slirp: Fix port comparision in slirp_remove_hostfwd

For UDP host forwardings, fport is not stable, every outgoing packet of
the redirection can modify it. Use getsockname instead to look up the
port that is actually used on the host side.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

slirp/slirp.c

@@ -761,12 +761,16 @@ int slirp_remove_hostfwd(int is_udp, int host_port)
 {
     struct socket *so;
     struct socket *head = (is_udp ? &udb : &tcb);
-    int fport = htons(host_port);
+    struct sockaddr_in addr;
+    int port = htons(host_port);
+    socklen_t addr_len;
     int n = 0;
 
  loop_again:
     for (so = head->so_next; so != head; so = so->so_next) {
-        if (so->so_fport == fport) {
+        addr_len = sizeof(addr);
+        if (getsockname(so->s, (struct sockaddr *)&addr, &addr_len) == 0 &&
+            addr.sin_port == port) {
             close(so->s);
             sofree(so);
             n++;