net, pidfs: enable handing out pidfds for reaped sk->sk_peer_pid

net, pidfs: enable handing out pidfds for reaped sk->sk_peer_pid

Now that all preconditions are met, allow handing out pidfs for reaped
sk->sk_peer_pids.

Thanks to Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
for pointing out that we need to limit this to AF_UNIX sockets for now.

Link: https://lore.kernel.org/20250425-work-pidfs-net-v2-4-450a19461e75@kernel.org
Reviewed-by: David Rheinsberg <david@readahead.eu>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>