dm-verity: use 2-way interleaved SHA-256 hashing when supported

When the crypto library provides an optimized implementation of
sha256_finup_2x(), use it to interleave the hashing of pairs of data
blocks.  On some CPUs this nearly doubles hashing performance.  The
increase in overall throughput of cold-cache dm-verity reads that I'm
seeing on arm64 and x86_64 is roughly 35% (though this metric is hard to
measure as it jumps around a lot).

For now this is done only on data blocks, not Merkle tree blocks.  We
could use sha256_finup_2x() on Merkle tree blocks too, but that is less
important as there aren't as many Merkle tree blocks as data blocks, and
that would require some additional code restructuring.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

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