Add CRC-32C support to fs-verity, to provide a faster alternative to
SHA-256 for users who want integrity-only (not authenticity), i.e. who
want to detect only accidental corruption, not malicious changes.

CRC-32C is chosen over CRC-32 because the CRC-32C polynomial is believed
to provide slightly better error-detection properties; and CRC-32C is
just as fast (or can be just as fast) as CRC-32, or even faster e.g. on
some x86 processors that have a CRC-32C instruction but not CRC-32.

We use "crc32c" from the crypto API, so the polynomial convention is
bitwise little-endian, the digest is bytewise little-endian, and the CRC
bits are inverted at the beginning and end (which is desirable).

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>