s390/pkey: fix PKEY_TYPE_EP11_AES handling in PKEY_KBLOB2PROTK[23]

Commit 'fa6999e3 ("s390/pkey: support CCA and EP11 secure ECC
private keys")' introduced a new PKEY_TYPE_EP11_AES type for the
PKEY_KBLOB2PROTK2 and a new IOCTL, PKEY_KBLOB2PROTK3, which both
allows userspace to convert opaque securekey blobs of this type into
protectedkey blobs. Unfortunately, all PKEY_KBLOB2PROTK2 and
PKEY_KBLOB2PROTK3 IOCTL requests with this keyblobs of this type
return with an error (-EINVAL). Fix PKEY_TYPE_EP11_AES handling in
PKEY_KBLOB2PROTK2 and PKEY_KBLOB2PROTK3 IOCTLs, so that userspace can
convert PKEY_TYPE_EP11_AES keyblobs into protectedkey blobs.

Add a helper function to decode the start and size of the internal
header as well as start and size of the keyblob payload of an existing
keyblob. Also validate the length of header and keyblob, as well as
the keyblob magic.

Introduce another helper function, which handles a raw key wrapping
request and do the keyblob decoding in the calling function. Remove
all other header-related calculations.

Fixes: fa6999e3 ("s390/pkey: support CCA and EP11 secure ECC private keys")
Signed-off-by: Holger Dengler <dengler@linux.ibm.com>
Reviewed-by: Ingo Franzki <ifranzki@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>