ip xfrm: Allow to specify "all" option for monitor

Just to be aligned with the usage output.
Signed-off-by: Vadim Kochan <vadim4j@gmail.com>

ip/xfrm_monitor.c

@@ -374,7 +374,7 @@ int do_xfrm_monitor(int argc, char **argv)
 			groups = 0;
 		} else if (matches(*argv, "help") == 0) {
 			usage();
-		} else {
+		} else if (strcmp(*argv, "all")) {
 			fprintf(stderr, "Argument \"%s\" is unknown, try \"ip xfrm monitor help\".\n", *argv);
 			exit(-1);
 		}