tools/nolibc: avoid error in dup2() if old fd equals new fd

dup2() allows both 'old' and 'new' to have the same value, which dup3()
does not. If libc dup2() is implemented through the dup3() system call,
then it would incorrectly fail in this case.

Avoid the error by handling old == new explicitly.

Fixes: 30ca205 ("tools headers: Move the nolibc header from rcutorture to tools/include/nolibc/")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250820-nolibc-dup2-einval-v2-1-807185a45c56@linutronix.de
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

Author: t-8ch

tools/include/nolibc/sys.h

@@ -238,6 +238,19 @@ static __attribute__((unused))
 int sys_dup2(int old, int new)
 {
 #if defined(__NR_dup3)
+	int ret, nr_fcntl;
+
+#ifdef __NR_fcntl64
+	nr_fcntl = __NR_fcntl64;
+#else
+	nr_fcntl = __NR_fcntl;
+#endif
+
+	if (old == new) {
+		ret = my_syscall2(nr_fcntl, old, F_GETFD);
+		return ret < 0 ? ret : old;
+	}
+
 	return my_syscall3(__NR_dup3, old, new, 0);
 #elif defined(__NR_dup2)
 	return my_syscall2(__NR_dup2, old, new);