af_unix: enable handing out pidfds for reaped tasks in SCM_PIDFD

Now everything is ready to pass PIDFD_STALE to pidfd_prepare().
This will allow opening pidfd for reaped tasks.

Cc: [e-mail]
Cc: [e-mail]
Cc: David S. Miller <[e-mail]>
Cc: Eric Dumazet <[e-mail]>
Cc: Jakub Kicinski <[e-mail]>
Cc: Paolo Abeni <[e-mail]>
Cc: Simon Horman <[e-mail]>
Cc: Willem de Bruijn <[e-mail]>
Cc: Leon Romanovsky <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Christian Brauner <[e-mail]>
Cc: Kuniyuki Iwashima <[e-mail]>
Cc: Lennart Poettering <[e-mail]>
Cc: Luca Boccassi <[e-mail]>
Cc: David Rheinsberg <[e-mail]>
Signed-off-by: Alexander Mikhalitsyn <[e-mail]>
Link: https://lore.kernel.org/[e-mail]
Reviewed-by: Christian Brauner <[e-mail]>
Reviewed-by: Kuniyuki Iwashima <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

net/core/scm.c

···
 #include <linux/security.h>
 #include <linux/pid_namespace.h>
 #include <linux/pid.h>
+#include <uapi/linux/pidfd.h>
 #include <linux/pidfs.h>
 #include <linux/nsproxy.h>
 #include <linux/slab.h>
···
 	if (!scm->pid)
 		return;
 
-	pidfd = pidfd_prepare(scm->pid, 0, &pidfd_file);
+	pidfd = pidfd_prepare(scm->pid, PIDFD_STALE, &pidfd_file);
 
 	if (put_cmsg(msg, SOL_SOCKET, SCM_PIDFD, sizeof(int), &pidfd)) {
 		if (pidfd_file) {