s390/crypto: fix gcc 8 stringop-truncation warning

Replace strncpy which is used to deliberately avoid string NUL-termination
with memcpy. This allows to get rid of gcc 8 stringop-truncation warning:

    inlined from 'query_crypto_facility.constprop' at
    drivers/s390/crypto/pkey_api.c:702:2:
./include/linux/string.h:246:9: warning: '__builtin_strncpy' output
truncated before terminating nul copying 8 bytes from a string of the
same length [-Wstringop-truncation]

Reviewed-by: Harald Freudenberger <freude@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>