crypto: dh - implement private key generation primitive for ffdheXYZ(dh)

The support for NVME in-band authentication currently in the works ([1])
needs to generate ephemeral DH keys for use with the RFC 7919 safe-prime
FFDHE groups.

In analogy to ECDH and its ecc_gen_privkey(), implement a
dh_safe_prime_gen_privkey() and invoke it from the ffdheXYZ(dh) templates'
common ->set_secret(), i.e. dh_safe_prime_set_secret(), in case the input
->key_size is zero.

As the RFC 7919 FFDHE groups are classified as approved safe-prime groups
by SP800-56Arev3, it's worthwhile to make the new
dh_safe_prime_gen_privkey() to follow the approach specified in
SP800-56Arev3, sec. 5.6.1.1.3 ("Key-Pair Generation Using Extra Random
Bits") in order to achieve conformance.

SP800-56Arev3 specifies a lower as well as an upper bound on the generated
key's length:
- it must be >= two times the maximum supported security strength of
  the group in question and
- it must be <= the length of the domain parameter Q.

For any safe-prime group Q = (P - 1)/2 by definition and the individual
maximum supported security strengths as specified by SP800-56Arev3 have
been made available as part of the FFDHE dh_safe_prime definitions
introduced with a previous patch. Make dh_safe_prime_gen_privkey() pick
twice the maximum supported strength rounded up to the next power of two
for the output key size. This choice respects both, the lower and upper
bounds given by SP800-90Arev3 for any of the approved safe-prime groups and
is also in line with the NVME base spec 2.0, which requires the key size to
be >= 256bits.

[1] https://lore.kernel.org/r/20211202152358.60116-1-hare@suse.de

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>

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