af_unix: don't post cmsg for SO_INQ unless explicitly asked for

A previous commit added SO_INQ support for AF_UNIX (SOCK_STREAM), but it
posts a SCM_INQ cmsg even if just msg->msg_get_inq is set. This is
incorrect, as ->msg_get_inq is just the caller asking for the remainder
to be passed back in msg->msg_inq, it has nothing to do with cmsg. The
original commit states that this is done to make sockets
io_uring-friendly", but it's actually incorrect as io_uring doesn't use
cmsg headers internally at all, and it's actively wrong as this means
that cmsg's are always posted if someone does recvmsg via io_uring.

Fix that up by only posting a cmsg if u->recvmsg_inq is set.

Additionally, mirror how TCP handles inquiry handling in that it should
only be done for a successful return. This makes the logic for the two
identical.

Cc: stable@vger.kernel.org
Fixes: df30285b3670 ("af_unix: Introduce SO_INQ.")
Reported-by: Julian Orth <ju.orth@gmail.com>
Link: https://github.com/axboe/liburing/issues/1509
Signed-off-by: Jens Axboe <axboe@kernel.dk>