btrfs: rename __btrfs_add_inode_defrag() and drop double underscores

The function does not follow the pattern where the underscores would be
justified, so rename it.

Also update the misleading comment, the passed item is not freed, that's
what the caller does.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>