crypto: xts - restrict key lengths to approved values in FIPS mode

According to FIPS 140-3 IG C.I., only (total) key lengths of either
256 bits or 512 bits are allowed with xts(aes). Make xts_verify_key() to
reject anything else in FIPS mode.

As xts(aes) is the only approved xts() template instantiation in FIPS mode,
the new restriction implemented in xts_verify_key() effectively only
applies to this particular construction.

Signed-off-by: Nicolai Stange <nstange@suse.de>
Signed-off-by: Vladis Dronov <vdronov@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>