btrfs: reloc: Fix NULL pointer dereference due to expanded reloc_root lifespan

Commit d2311e69 ("btrfs: relocation: Delay reloc tree deletion after
merge_reloc_roots()") expands the life span of root->reloc_root.

This breaks certain checs of fs_info->reloc_ctl.  Before that commit, if
we have a root with valid reloc_root, then it's ensured to have
fs_info->reloc_ctl.

But now since reloc_root doesn't always mean a valid fs_info->reloc_ctl,
such check is unreliable and can cause the following NULL pointer
dereference:

  BUG: unable to handle kernel NULL pointer dereference at 00000000000005c1
  IP: btrfs_reloc_pre_snapshot+0x20/0x50 [btrfs]
  PGD 0 P4D 0
  Oops: 0000 [#1] SMP PTI
  CPU: 0 PID: 10379 Comm: snapperd Not tainted
  Call Trace:
   create_pending_snapshot+0xd7/0xfc0 [btrfs]
   create_pending_snapshots+0x8e/0xb0 [btrfs]
   btrfs_commit_transaction+0x2ac/0x8f0 [btrfs]
   btrfs_mksubvol+0x561/0x570 [btrfs]
   btrfs_ioctl_snap_create_transid+0x189/0x190 [btrfs]
   btrfs_ioctl_snap_create_v2+0x102/0x150 [btrfs]
   btrfs_ioctl+0x5c9/0x1e60 [btrfs]
   do_vfs_ioctl+0x90/0x5f0
   SyS_ioctl+0x74/0x80
   do_syscall_64+0x7b/0x150
   entry_SYSCALL_64_after_hwframe+0x3d/0xa2
  RIP: 0033:0x7fd7cdab8467

Fix it by explicitly checking fs_info->reloc_ctl other than using the
implied root->reloc_root.

Fixes: d2311e69 ("btrfs: relocation: Delay reloc tree deletion after merge_reloc_roots")
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>