selftest: af_unix: Add test for SO_INQ.

Let's add a simple test to check the basic functionality of SO_INQ.

The test does the following:

  1. Create socketpair in self->fd[]
  2. Enable SO_INQ
  3. Send data via self->fd[0]
  4. Receive data from self->fd[1]
  5. Compare the SCM_INQ cmsg with ioctl(SIOCINQ)

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250702223606.1054680-8-kuniyu@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>