s390/zcrypt: use kvmalloc_array() instead of kzalloc()

zcrypt_unlocked_ioctl() allocates 256k with kzalloc() which is likely to
fail if memory is fragmented. To avoid that use kvmalloc_array() instead,
like it is done at several other places for the same reason.

Reviewed-by: Harald Freudenberger <freude@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>