s390/pkey: Rework pkey verify for protected keys

Rework the verification of protected keys by simple check
for the correct AES wrapping key verification pattern.

A protected key always carries the AES wrapping key
verification pattern within the blob. The old code really
used the protected key for an en/decrypt operation and by
doing so, verified the AES WK VP. But a much simpler and
more generic way is to extract the AES WK VP value from the
key and compare it with AES WK VP from a freshly created
dummy protected key. This also eliminates the limitation to
only be able to verify AES protected keys. With this change
any kind of known protected key can be verified.

Suggested-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>