s390/pkey: use memdup_user() to simplify code

Generated by: scripts/coccinelle/api/memdup_user.cocci

Link: http://lkml.kernel.org/r/aca044e8-e4b2-eda8-d724-b08772a44ed9@web.de


[borntraeger@de.ibm.com: use ==0 instead of <=0 for a size_t variable]
[heiko.carstens@de.ibm.com: split bugfix into separate patch; shorten changelog]
Signed-off-by: Markus Elfring <Markus.Elfring@web.de>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>