btrfs: Add missing lock annotation for release_extent_buffer()

Sparse reports a warning at release_extent_buffer()
warning: context imbalance in release_extent_buffer() - unexpected unlock

The root cause is the missing annotation at release_extent_buffer()
Add the missing __releases(&eb->refs_lock) annotation

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>