epoll: Always use epoll_ref for the epoll data variable

epoll_ref contains a variety of information useful when handling epoll
events on our sockets, and we place it in the epoll_event data field
returned by epoll.  However, for a few other things we use the 'fd' field
in the standard union of types for that data field.

This actually introduces a bug which is vanishingly unlikely to hit in
practice, but very nasty if it ever did: theoretically if we had a very
large file descriptor number for fd_tap or fd_tap_listen it could overflow
into bits that overlap with the 'proto' field in epoll_ref.  With some
very bad luck this could mean that we mistakenly think an event on a
regular socket is an event on fd_tap or fd_tap_listen.

More practically, using different (but overlapping) fields of the
epoll_data means we can't unify dispatch for the various different objects
in the epoll.  Therefore use the same epoll_ref as the data for the tap
fds and the netns quit fd, adding new fd type values to describe them.

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

passt.h

@@ -55,8 +55,12 @@ enum epoll_type {
 	EPOLL_TYPE_ICMP,
 	/* ICMPv6 sockets */
 	EPOLL_TYPE_ICMPV6,
+	/* inotify fd watching for end of netns (pasta) */
+	EPOLL_TYPE_NSQUIT,
+	/* tap char device, or qemu socket fd */
+	EPOLL_TYPE_TAP,
 
-	EPOLL_TYPE_MAX = EPOLL_TYPE_ICMPV6,
+	EPOLL_TYPE_MAX = EPOLL_TYPE_TAP,
 };
 
 /**