s390/pkey: add missing __user annotations

Add missing __user annotations to get rid of a couple of sparse
warnings. All callers actually pass kernel pointers instead of user
space pointers, however the pointers are being used within
KERNEL_DS. So everything is fine.

Corresponding sparse warnings:

drivers/s390/crypto/pkey_api.c:181:41:
 warning: incorrect type in assignment (different address spaces)
    expected char [noderef] <asn:1>*request_control_blk_addr
    got void *<noident>

Cc: Harald Freudenberger <freude@linux.vnet.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>