opencrypto: Disallow requests which pass VERIFY_DIGEST without a MAC

[markj]

Summary

Otherwise we can end up comparing the computed digest with an
uninitialized kernel buffer.

In cryptoaead_op() we already unconditionally fail the request if a
pointer to a digest buffer is not specified.

Based on a patch by Simran Kathpalia.

Reported by: syzkaller

[jhb]

So in the case that cop->mac is NULL, the copyin should fail with EFAULT? Ah, yes, and this is what we want, yes.