powerpc/crypto: Add cond_resched() in crc-vpmsum self-test

The stress test for vpmsum implementations executes a long for loop in
the kernel. This blocks the scheduler, which prevents other tasks from
running, resulting in a warning.

This fix adds a call to cond_reshed() at the end of each loop, which
allows the scheduler to run other tasks as required.

Signed-off-by: Chris Smart <chris.smart@humanservices.gov.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191103233356.5472-1-chris.smart@humanservices.gov.au