nbd: Shutdown socket before closing.

This forces finishing data sending to client before closing the socket like in
exports listing or replying with NBD_REP_ERR_UNSUP cases.

Signed-off-by: Hani Benhabiles <kroosec@gmail.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

qemu-nbd.c

@@ -372,6 +372,7 @@ static void nbd_accept(void *opaque)
     if (nbd_client_new(exp, fd, nbd_client_closed)) {
         nb_fds++;
     } else {
+        shutdown(fd, 2);
         close(fd);
     }
 }