btrfs_decompress_bio, the only caller of compression_decompress_bio gets
type from @cb and passes it to compression_decompress_bio.
However, compression_decompress_bio can get compression type directly
from @cb.

So remove the parameter and access it through @cb.  No functional
change.
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Su Yue <l@damenly.su>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>