crypto: dh - disallow plain "dh" usage in FIPS mode

SP800-56Arev3, sec. 5.5.2 ("Assurance of Domain-Parameter Validity")
asserts that an implementation needs to verify domain paramtere validity,
which boils down to either
- the domain parameters corresponding to some known safe-prime group
  explicitly listed to be approved in the document or
- for parameters conforming to a "FIPS 186-type parameter-size set",
  that the implementation needs to perform an explicit domain parameter
  verification, which would require access to the "seed" and "counter"
  values used in their generation.

The latter is not easily feasible and moreover, SP800-56Arev3 states that
safe-prime groups are preferred and that FIPS 186-type parameter sets
should only be supported for backward compatibility, if it all.

Mark "dh" as not fips_allowed in testmgr. Note that the safe-prime
ffdheXYZ(dh) wrappers are not affected by this change: as these enforce
some approved safe-prime group each, their usage is still allowed in FIPS
mode.

This change will effectively render the keyctl(KEYCTL_DH_COMPUTE) syscall
unusable in FIPS mode, but it has been brought up that this might even be
a good thing ([1]).

[1] https://lore.kernel.org/r/20211217055227.GA20698@gondor.apana.org.au

Signed-off-by: Nicolai Stange <nstange@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/testmgr.c

@@ -4650,7 +4650,6 @@ static const struct alg_test_desc alg_test_descs[] = {
 	}, {
 		.alg = "dh",
 		.test = alg_test_kpp,
-		.fips_allowed = 1,
 		.suite = {
 			.kpp = __VECS(dh_tv_template)
 		}