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>