btrfs: make load_block_group_size_class() return void

btrfs: make load_block_group_size_class() return void

There's no point in returning anything since determining and setting a
size class for a block group is an optimization, not something critical.
The only caller of load_block_group_size_class() (the caching thread)
does not do anything with the return value anyway, exactly because having
a size class is just an optimization and it can always be set later when
adding reserved bytes to a block group (btrfs_add_reserved_bytes()).

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>