selftests/pidfd: fixes syscall number defines

I had to spend some (a lot;) time to understand why pidfd_info_test
(and more) fails with my patch under qemu on my machine ;) Until I
applied the patch below.

I think it is a bad idea to do the things like

	#ifndef __NR_clone3
	#define __NR_clone3 -1
	#endif

because this can hide a problem. My working laptop runs Fedora-23 which
doesn't have __NR_clone3/etc in /usr/include/. So "make" happily succeeds,
but everything fails and it is not clear why.

Link: https://lore.kernel.org/r/20250323174518.GB834@redhat.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: oleg-nesterov

tools/testing/selftests/clone3/clone3_selftests.h

@@ -16,7 +16,7 @@
 #define ptr_to_u64(ptr) ((__u64)((uintptr_t)(ptr)))
 
 #ifndef __NR_clone3
-#define __NR_clone3 -1
+#define __NR_clone3 435
 #endif
 
 struct __clone_args {

tools/testing/selftests/pidfd/pidfd.h

@@ -32,19 +32,19 @@
 #endif
 
 #ifndef __NR_pidfd_open
-#define __NR_pidfd_open -1
+#define __NR_pidfd_open 434
 #endif
 
 #ifndef __NR_pidfd_send_signal
-#define __NR_pidfd_send_signal -1
+#define __NR_pidfd_send_signal 424
 #endif
 
 #ifndef __NR_clone3
-#define __NR_clone3 -1
+#define __NR_clone3 435
 #endif
 
 #ifndef __NR_pidfd_getfd
-#define __NR_pidfd_getfd -1
+#define __NR_pidfd_getfd 438
 #endif
 
 #ifndef PIDFD_NONBLOCK