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Add a slicing-by-8 CRC32c implementation for metadata checksumming.
Adapted from Bob Pearson's kernel patch.

Also added a self-test mechanism so we can verify that the crc32c
implementation is working correctly.
Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>