btrfs: fix data race at btrfs_use_block_rsv() when accessing block reserve

At btrfs_use_block_rsv() we read the size of a block reserve without
locking its spinlock, which makes KCSAN complain because the size of a
block reserve is always updated while holding its spinlock. The report
from KCSAN is the following:

  [653.313148] BUG: KCSAN: data-race in btrfs_update_delayed_refs_rsv [btrfs] / btrfs_use_block_rsv [btrfs]

  [653.314755] read to 0x000000017f5871b8 of 8 bytes by task 7519 on cpu 0:
  [653.314779]  btrfs_use_block_rsv+0xe4/0x2f8 [btrfs]
  [653.315606]  btrfs_alloc_tree_block+0xdc/0x998 [btrfs]
  [653.316421]  btrfs_force_cow_block+0x220/0xe38 [btrfs]
  [653.317242]  btrfs_cow_block+0x1ac/0x568 [btrfs]
  [653.318060]  btrfs_search_slot+0xda2/0x19b8 [btrfs]
  [653.318879]  btrfs_del_csums+0x1dc/0x798 [btrfs]
  [653.319702]  __btrfs_free_extent.isra.0+0xc24/0x2028 [btrfs]
  [653.320538]  __btrfs_run_delayed_refs+0xd3c/0x2390 [btrfs]
  [653.321340]  btrfs_run_delayed_refs+0xae/0x290 [btrfs]
  [653.322140]  flush_space+0x5e4/0x718 [btrfs]
  [653.322958]  btrfs_preempt_reclaim_metadata_space+0x102/0x2f8 [btrfs]
  [653.323781]  process_one_work+0x3b6/0x838
  [653.323800]  worker_thread+0x75e/0xb10
  [653.323817]  kthread+0x21a/0x230
  [653.323836]  __ret_from_fork+0x6c/0xb8
  [653.323855]  ret_from_fork+0xa/0x30

  [653.323887] write to 0x000000017f5871b8 of 8 bytes by task 576 on cpu 3:
  [653.323906]  btrfs_update_delayed_refs_rsv+0x1a4/0x250 [btrfs]
  [653.324699]  btrfs_add_delayed_data_ref+0x468/0x6d8 [btrfs]
  [653.325494]  btrfs_free_extent+0x76/0x120 [btrfs]
  [653.326280]  __btrfs_mod_ref+0x6a8/0x6b8 [btrfs]
  [653.327064]  btrfs_dec_ref+0x50/0x70 [btrfs]
  [653.327849]  walk_up_proc+0x236/0xa50 [btrfs]
  [653.328633]  walk_up_tree+0x21c/0x448 [btrfs]
  [653.329418]  btrfs_drop_snapshot+0x802/0x1328 [btrfs]
  [653.330205]  btrfs_clean_one_deleted_snapshot+0x184/0x238 [btrfs]
  [653.330995]  cleaner_kthread+0x2b0/0x2f0 [btrfs]
  [653.331781]  kthread+0x21a/0x230
  [653.331800]  __ret_from_fork+0x6c/0xb8
  [653.331818]  ret_from_fork+0xa/0x30

So add a helper to get the size of a block reserve while holding the lock.
Reading the field while holding the lock instead of using the data_race()
annotation is used in order to prevent load tearing.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>