s390/extmem: fix gcc 8 stringop-overflow warning

arch/s390/mm/extmem.c: In function '__segment_load':
arch/s390/mm/extmem.c:436:2: warning: 'strncat' specified bound 7 equals
source length [-Wstringop-overflow=]
  strncat(seg->res_name, " (DCSS)", 7);

What gcc complains about here is the misuse of strncat function, which
in this case does not limit a number of bytes taken from "src", so it is
in the end the same as strcat(seg->res_name, " (DCSS)");

Keeping in mind that a res_name is 15 bytes, strncat in this case
would overflow the buffer and write 0 into alignment byte between the
fields in the struct. To avoid that increasing res_name size to 16,
and reusing strlcat.

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>

arch/s390/mm/extmem.c

@@ -80,7 +80,7 @@ struct qin64 {
 struct dcss_segment {
 	struct list_head list;
 	char dcss_name[8];
-	char res_name[15];
+	char res_name[16];
 	unsigned long start_addr;
 	unsigned long end;
 	atomic_t ref_count;
@@ -433,7 +433,7 @@ __segment_load (char *name, int do_nonshared, unsigned long *addr, unsigned long
 	memcpy(&seg->res_name, seg->dcss_name, 8);
 	EBCASC(seg->res_name, 8);
 	seg->res_name[8] = '\0';
-	strncat(seg->res_name, " (DCSS)", 7);
+	strlcat(seg->res_name, " (DCSS)", sizeof(seg->res_name));
 	seg->res->name = seg->res_name;
 	rc = seg->vm_segtype;
 	if (rc == SEG_TYPE_SC ||