selftests: watchdog: Validate optional file argument

The newly added optional file argument does not validate if the
file is indeed a watchdog, e.g.:

./watchdog-test -f /dev/zero
Watchdog Ticking Away!

Fix it by confirming that the WDIOC_GETSUPPORT ioctl succeeds.

Fixes: a4864a33f56caa ("selftests: watchdog: Add optional file argument")
Reported-by: Eugeniu Rosca <[e-mail]>
Signed-off-by: George G. Davis <[e-mail]>
Signed-off-by: Eugeniu Rosca <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/watchdog/watchdog-test.c

···
 	int c;
 	int oneshot = 0;
 	char *file = "/dev/watchdog";
+	struct watchdog_info info;
 
 	setbuf(stdout, NULL);
 
···
 			printf("Watchdog device open failed %s\n",
 				strerror(errno));
 		exit(-1);
+	}
+
+	/*
+	 * Validate that `file` is a watchdog device
+	 */
+	ret = ioctl(fd, WDIOC_GETSUPPORT, &info);
+	if (ret) {
+		printf("WDIOC_GETSUPPORT error '%s'\n", strerror(errno));
+		close(fd);
+		exit(ret);
 	}
 
 	optind = 0;