af_unix: Don't use skb_recv_datagram() in unix_stream_read_skb().

unix_stream_read_skb() calls skb_recv_datagram() with MSG_DONTWAIT,
which is mostly equivalent to sock_error(sk) + skb_dequeue().

In the following patch, we will add a new field to cache the number
of bytes in the receive queue. Then, we want to avoid introducing
atomic ops in the fast path, so we will reuse the receive queue lock.

As a preparation for the change, let's not use skb_recv_datagram()
in unix_stream_read_skb().

Note that sock_error() is now moved out of the u->iolock mutex as
the mutex does not synchronise the peer's close() at all.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250702223606.1054680-4-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>