s390/dasd: fix gcc 8 stringop-truncation warning

drivers/s390/block/dasd_alias.c:711:2: warning: 'strncpy' output truncated
before terminating nul copying 4 bytes from a string of the same length
[-Wstringop-truncation]
  strncpy((char *) &cqr->magic, "ECKD", 4);

Replace strncpy to int as a buffer with memcpy.
Acked-by: Stefan Haberland <sth@linux.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/block/dasd_alias.c

@@ -708,7 +708,7 @@ static int reset_summary_unit_check(struct alias_lcu *lcu,
 	struct ccw1 *ccw;
 
 	cqr = lcu->rsu_cqr;
-	strncpy((char *) &cqr->magic, "ECKD", 4);
+	memcpy((char *) &cqr->magic, "ECKD", 4);
 	ASCEBC((char *) &cqr->magic, 4);
 	ccw = cqr->cpaddr;
 	ccw->cmd_code = DASD_ECKD_CCW_RSCK;