ceph: fix incorrect use of strncpy

GCC8 prints following warning:

 fs/ceph/mds_client.c:3683:2: warning: ‘strncpy’ output may be truncated
 copying 64 bytes from a string of length 64 [-Wstringop-truncation]

[ Change to strscpy() while at it. ]
Signed-off-by: N"Yan, Zheng" <zyan@redhat.com>
Reviewed-by: NIlya Dryomov <idryomov@gmail.com>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>

fs/ceph/mds_client.c

@@ -3644,8 +3644,8 @@ int ceph_mdsc_init(struct ceph_fs_client *fsc)
 	init_rwsem(&mdsc->pool_perm_rwsem);
 	mdsc->pool_perm_tree = RB_ROOT;
 
-	strncpy(mdsc->nodename, utsname()->nodename,
-		sizeof(mdsc->nodename) - 1);
+	strscpy(mdsc->nodename, utsname()->nodename,
+		sizeof(mdsc->nodename));
 	return 0;
 }