FROMLIST: fsverity: improve performance by using multibuffer hashing

When supported by the hash algorithm, use crypto_shash_finup_mb() 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 fsverity 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 only done on the verification path, and only for data
blocks, not Merkle tree blocks.  We could use finup_mb 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.  We could also use finup_mb to accelerate building the
Merkle tree, but verification performance is more important.

Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>

Bug: 330611177
Link: https://lore.kernel.org/r/20240621165922.77672-7-ebiggers@kernel.org


Change-Id: Ib5de1fba462825004f46ed0fb344c9a89603405b
Signed-off-by: Eric Biggers <ebiggers@google.com>