selftests/nolibc: create /dev/full when running as PID 1

An upcoming testcase will use /dev/full.

Make sure it is always present.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250704-nolibc-printf-error-v1-1-74b7a092433b@linutronix.de
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

Author: t-8ch

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

@@ -1778,12 +1778,14 @@ int prepare(void)
 	if (stat("/dev/.", &stat_buf) == 0 || mkdir("/dev", 0755) == 0) {
 		if (stat("/dev/console", &stat_buf) != 0 ||
 		    stat("/dev/null", &stat_buf) != 0 ||
-		    stat("/dev/zero", &stat_buf) != 0) {
+		    stat("/dev/zero", &stat_buf) != 0 ||
+		    stat("/dev/full", &stat_buf) != 0) {
 			/* try devtmpfs first, otherwise fall back to manual creation */
 			if (mount("/dev", "/dev", "devtmpfs", 0, 0) != 0) {
 				mknod("/dev/console", 0600 | S_IFCHR, makedev(5, 1));
 				mknod("/dev/null",    0666 | S_IFCHR, makedev(1, 3));
 				mknod("/dev/zero",    0666 | S_IFCHR, makedev(1, 5));
+				mknod("/dev/full",    0666 | S_IFCHR, makedev(1, 7));
 			}
 		}
 	}