f2fs: remove unused sbi argument from checksum functions

Since __f2fs_crc32() now calls crc32() directly, it no longer uses its
sbi argument.  Remove that, and simplify its callers accordingly.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>