btrfs: don't pass block group argument to load_block_group_size_class()

btrfs: don't pass block group argument to load_block_group_size_class()

There's no need to pass the block group since we can extract it from
the given caching control structure. Same goes for its helper function
sample_block_group_extent_item().

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