s390/pkey: Check length in PKEY_VERIFYPROTK ioctl

s390/pkey: Check length in PKEY_VERIFYPROTK ioctl

Explicitly check the buffer length request structure provided by
user-space and fail, if it exceeds the buffer size.

Cc: stable@vger.kernel.org
Fixes: 8fcc231ce3be ("s390/pkey: Introduce pkey base with handler registry and handler modules")
Reported-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Reviewed-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Ingo Franzki <ifranzki@linux.ibm.com>
Signed-off-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>