icmp: Work around possible failure on bind() due to e.g. broken SELinux policy

If we can't bind() ping sockets, the echo identifier sent out from
the socket won't be the original one seen from the tap. Binding a
ping socket doesn't require any security capability, but it might
still fail due to a broken SELinux policy, see for example:
	https://bugzilla.redhat.com/show_bug.cgi?id=1848929

Track the ICMP echo identifier as part of the epoll reference for
the socket and replace it in the reply on mismatch. We won't send
out the original identifier as sent from the guest, but still better
than missing replies.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

icmp.h

@@ -15,10 +15,12 @@ void icmp_timer(struct ctx *c, struct timespec *ts);
  * union icmp_epoll_ref - epoll reference portion for ICMP tracking
  * @v6:			Set for IPv6 sockets or connections
  * @u32:		Opaque u32 value of reference
+ * @id:			Associated echo identifier, needed if bind() fails
  */
 union icmp_epoll_ref {
 	struct {
-		uint32_t	v6:1;
+		uint32_t	v6:1,
+				id:16;
 	};
 	uint32_t u32;
 };