af_unix: Don't set -ECONNRESET for consumed OOB skb.

af_unix: Don't set -ECONNRESET for consumed OOB skb.

Christian Brauner reported that even after MSG_OOB data is consumed,
calling close() on the receiver socket causes the peer's recv() to
return -ECONNRESET:

  1. send() and recv() an OOB data.

    >>> from socket import *
    >>> s1, s2 = socketpair(AF_UNIX, SOCK_STREAM)
    >>> s1.send(b'x', MSG_OOB)
    1
    >>> s2.recv(1, MSG_OOB)
    b'x'

  2. close() for s2 sets ECONNRESET to s1->sk_err even though
     s2 consumed the OOB data

    >>> s2.close()
    >>> s1.recv(10, MSG_DONTWAIT)
    ...
    ConnectionResetError: [Errno 104] Connection reset by peer

Even after being consumed, the skb holding the OOB 1-byte data stays in
the recv queue to mark the OOB boundary and break recv() at that point.

This must be considered while close()ing a socket.

Let's skip the leading consumed OOB skb while checking the -ECONNRESET
condition in unix_release_sock().

Fixes: 314001f0bf92 ("af_unix: Add OOB support")
Reported-by: Christian Brauner <brauner@kernel.org>
Closes: https://lore.kernel.org/netdev/20250529-sinkt-abfeuern-e7b08200c6b0@brauner/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Acked-by: Christian Brauner <brauner@kernel.org>
Link: https://patch.msgid.link/20250619041457.1132791-4-kuni1840@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>