s390/pkey: fix kzalloc-simple.cocci warnings

drivers/s390/crypto/pkey_api.c:128:11-18: WARNING:
kzalloc should be used for cprbmem, instead of kmalloc/memset

Use kzalloc rather than kmalloc followed by memset with 0

Signed-off-by: Vasyl Gomonovych <gomonovych@gmail.com>
Signed-off-by: Harald Freudenberger <freude@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>