af_unix: stash pidfs dentry when needed

We need to ensure that pidfs dentry is allocated when we meet any
struct pid for the first time. This will allows us to open pidfd
even after the task it corresponds to is reaped.

Basically, we need to identify all places where we fill skb/scm_cookie
with struct pid reference for the first time and call pidfs_register_pid().

Tricky thing here is that we have a few places where this happends
depending on what userspace is doing:
- [__scm_replace_pid()] explicitly sending an SCM_CREDENTIALS message
                        and specified pid in a numeric format
- [unix_maybe_add_creds()] enabled SO_PASSCRED/SO_PASSPIDFD but
                           didn't send SCM_CREDENTIALS explicitly
- [scm_send()] force_creds is true. Netlink case, we don't need to touch it.

Cc: linux-kernel@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: David S. Miller <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Simon Horman <horms@kernel.org>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Kuniyuki Iwashima <kuniyu@google.com>
Cc: Lennart Poettering <mzxreary@0pointer.de>
Cc: Luca Boccassi <bluca@debian.org>
Cc: David Rheinsberg <david@readahead.eu>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Link: https://lore.kernel.org/20250703222314.309967-6-aleksandr.mikhalitsyn@canonical.com
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

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