cipher: Allow generation of RSA keys > 2k

• cipher/rsa.c (generate_fips): Allow any larger key than 2k in FIPS mode. --

The NIST.SP.800-131Ar2, Table 2 explicitly mentions the approved RSA
sizes to be:

len(n) ≥ 2048 to be

On the other hand, older standard NIST.FIPS.186-4, section 5.1 describes
only 1024, 2048 and 3072 bits sizes and Federal government shall used
one of these.

But the NIST.FIPS.186-5 draft already clarified that

This standard specifies the use of a modulus whose bit length is an
even integer and greater than or equal to 2048 bits.

• GnuPG-bug-id: T5645
• Signed-off-by: Jakub Jelen <jjelen@redhat.com>