crypto: testmgr - Add multibuffer hash testing

This is based on a patch by Eric Biggers <ebiggers@google.com>.

Add limited self-test for multibuffer hash code path.  This tests
only a single request in chain of a random length.  The other
requests are either all of the same length as the one being tested,
or random lengths between 0 and PAGE_SIZE * 2 * XBUFSIZE.

Potential extension include testing all requests rather than just
the single one.

Link: https://lore.kernel.org/all/20241001153718.111665-3-ebiggers@kernel.org/


Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>