btrfs: fix lockdep splat in add_missing_dev

Nikolay reported a lockdep splat in generic/476 that I could reproduce
with btrfs/187.

  ======================================================
  WARNING: possible circular locking dependency detected
  5.9.0-rc2+ #1 Tainted: G        W
  ------------------------------------------------------
  kswapd0/100 is trying to acquire lock:
  ffff9e8ef38b6268 (&delayed_node->mutex){+.+.}-{3:3}, at: __btrfs_release_delayed_node.part.0+0x3f/0x330

  but task is already holding lock:
  ffffffffa9d74700 (fs_reclaim){+.+.}-{0:0}, at: __fs_reclaim_acquire+0x5/0x30

  which lock already depends on the new lock.

  the existing dependency chain (in reverse order) is:

  -> #2 (fs_reclaim){+.+.}-{0:0}:
	 fs_reclaim_acquire+0x65/0x80
	 slab_pre_alloc_hook.constprop.0+0x20/0x200
	 kmem_cache_alloc_trace+0x3a/0x1a0
	 btrfs_alloc_device+0x43/0x210
	 add_missing_dev+0x20/0x90
	 read_one_chunk+0x301/0x430
	 btrfs_read_sys_array+0x17b/0x1b0
	 open_ctree+0xa62/0x1896
	 btrfs_mount_root.cold+0x12/0xea
	 legacy_get_tree+0x30/0x50
	 vfs_get_tree+0x28/0xc0
	 vfs_kern_mount.part.0+0x71/0xb0
	 btrfs_mount+0x10d/0x379
	 legacy_get_tree+0x30/0x50
	 vfs_get_tree+0x28/0xc0
	 path_mount+0x434/0xc00
	 __x64_sys_mount+0xe3/0x120
	 do_syscall_64+0x33/0x40
	 entry_SYSCALL_64_after_hwframe+0x44/0xa9

  -> #1 (&fs_info->chunk_mutex){+.+.}-{3:3}:
	 __mutex_lock+0x7e/0x7e0
	 btrfs_chunk_alloc+0x125/0x3a0
	 find_free_extent+0xdf6/0x1210
	 btrfs_reserve_extent+0xb3/0x1b0
	 btrfs_alloc_tree_block+0xb0/0x310
	 alloc_tree_block_no_bg_flush+0x4a/0x60
	 __btrfs_cow_block+0x11a/0x530
	 btrfs_cow_block+0x104/0x220
	 btrfs_search_slot+0x52e/0x9d0
	 btrfs_lookup_inode+0x2a/0x8f
	 __btrfs_update_delayed_inode+0x80/0x240
	 btrfs_commit_inode_delayed_inode+0x119/0x120
	 btrfs_evict_inode+0x357/0x500
	 evict+0xcf/0x1f0
	 vfs_rmdir.part.0+0x149/0x160
	 do_rmdir+0x136/0x1a0
	 do_syscall_64+0x33/0x40
	 entry_SYSCALL_64_after_hwframe+0x44/0xa9

  -> #0 (&delayed_node->mutex){+.+.}-{3:3}:
	 __lock_acquire+0x1184/0x1fa0
	 lock_acquire+0xa4/0x3d0
	 __mutex_lock+0x7e/0x7e0
	 __btrfs_release_delayed_node.part.0+0x3f/0x330
	 btrfs_evict_inode+0x24c/0x500
	 evict+0xcf/0x1f0
	 dispose_list+0x48/0x70
	 prune_icache_sb+0x44/0x50
	 super_cache_scan+0x161/0x1e0
	 do_shrink_slab+0x178/0x3c0
	 shrink_slab+0x17c/0x290
	 shrink_node+0x2b2/0x6d0
	 balance_pgdat+0x30a/0x670
	 kswapd+0x213/0x4c0
	 kthread+0x138/0x160
	 ret_from_fork+0x1f/0x30

  other info that might help us debug this:

  Chain exists of:
    &delayed_node->mutex --> &fs_info->chunk_mutex --> fs_reclaim

   Possible unsafe locking scenario:

	 CPU0                    CPU1
	 ----                    ----
    lock(fs_reclaim);
				 lock(&fs_info->chunk_mutex);
				 lock(fs_reclaim);
    lock(&delayed_node->mutex);

   *** DEADLOCK ***

  3 locks held by kswapd0/100:
   #0: ffffffffa9d74700 (fs_reclaim){+.+.}-{0:0}, at: __fs_reclaim_acquire+0x5/0x30
   #1: ffffffffa9d65c50 (shrinker_rwsem){++++}-{3:3}, at: shrink_slab+0x115/0x290
   #2: ffff9e8e9da260e0 (&type->s_umount_key#48){++++}-{3:3}, at: super_cache_scan+0x38/0x1e0

  stack backtrace:
  CPU: 1 PID: 100 Comm: kswapd0 Tainted: G        W         5.9.0-rc2+ #1
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-2.fc32 04/01/2014
  Call Trace:
   dump_stack+0x92/0xc8
   check_noncircular+0x12d/0x150
   __lock_acquire+0x1184/0x1fa0
   lock_acquire+0xa4/0x3d0
   ? __btrfs_release_delayed_node.part.0+0x3f/0x330
   __mutex_lock+0x7e/0x7e0
   ? __btrfs_release_delayed_node.part.0+0x3f/0x330
   ? __btrfs_release_delayed_node.part.0+0x3f/0x330
   ? lock_acquire+0xa4/0x3d0
   ? btrfs_evict_inode+0x11e/0x500
   ? find_held_lock+0x2b/0x80
   __btrfs_release_delayed_node.part.0+0x3f/0x330
   btrfs_evict_inode+0x24c/0x500
   evict+0xcf/0x1f0
   dispose_list+0x48/0x70
   prune_icache_sb+0x44/0x50
   super_cache_scan+0x161/0x1e0
   do_shrink_slab+0x178/0x3c0
   shrink_slab+0x17c/0x290
   shrink_node+0x2b2/0x6d0
   balance_pgdat+0x30a/0x670
   kswapd+0x213/0x4c0
   ? _raw_spin_unlock_irqrestore+0x46/0x60
   ? add_wait_queue_exclusive+0x70/0x70
   ? balance_pgdat+0x670/0x670
   kthread+0x138/0x160
   ? kthread_create_worker_on_cpu+0x40/0x40
   ret_from_fork+0x1f/0x30

This is because we are holding the chunk_mutex when we call
btrfs_alloc_device, which does a GFP_KERNEL allocation.  We don't want
to switch that to a GFP_NOFS lock because this is the only place where
it matters.  So instead use memalloc_nofs_save() around the allocation
in order to avoid the lockdep splat.

Reported-by: Nikolay Borisov <nborisov@suse.com>
CC: stable@vger.kernel.org # 4.4+
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>