btrfs: block-group: rename write_one_cache_group()

The name of this function contains the word "cache", which is left from
the times where btrfs_block_group was called btrfs_block_group_cache.

Now this "cache" doesn't match anything, and we have better namings for
functions like read/insert/remove_block_group_item().

Rename it to update_block_group_item().

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>