btrfs: add missing annotation for btrfs_tree_lock()

Sparse reports a warning at btrfs_tree_lock()

warning: context imbalance in btrfs_tree_lock() - wrong count at exit

The root cause is the missing annotation at btrfs_tree_lock()
Add the missing __acquires(&eb->lock) annotation
Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/locking.c

@@ -410,6 +410,7 @@ void btrfs_tree_read_unlock_blocking(struct extent_buffer *eb)
  * The rwlock is held for write upon exit.
  */
 void btrfs_tree_lock(struct extent_buffer *eb)
+	__acquires(&eb->lock)
 {
 	u64 start_ns = 0;