epoll: Split handling of TCP timerfds into its own handler function

tcp_sock_handler() actually handles several different types of fd events.
This includes timerfds that aren't sockets at all.  The handling of these
has essentially nothing in common with the other cases.  So, give the
TCP timers there own epoll_type value and dispatch directly to their
handler.  This also means we can remove the timer field from tcp_epoll_ref,
the information it encoded is now implicit in the epoll_type value.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

passt.h

@@ -47,8 +47,10 @@ union epoll_ref;
 enum epoll_type {
 	/* Special value to indicate an invalid type */
 	EPOLL_TYPE_NONE = 0,
-	/* Sockets and timerfds for TCP handling */
+	/* TCP sockets */
 	EPOLL_TYPE_TCP,
+	/* timerfds used for TCP timers */
+	EPOLL_TYPE_TCP_TIMER,
 	/* UDP sockets */
 	EPOLL_TYPE_UDP,
 	/* IPv4 ICMP sockets */