flow,tcp: Use epoll_ref type including flow and side

Currently TCP uses the 'flow' epoll_ref field for both connected
sockets and timers, which consists of just the index of the relevant
flow (connection).

This is just fine for timers, for while it obviously works, it's
subtly incomplete for sockets on spliced connections.  In that case we
want to know which side of the connection the event is occurring on as
well as which connection.  At present, we deduce that information by
looking at the actual fd, and comparing it to the fds of the sockets
on each side.

When we use the flow table for more things, we expect more cases where
something will need to know a specific side of a specific flow for an
event, but nothing more.

Therefore add a new 'flowside' epoll_ref field, with exactly that
information.  We use it for TCP connected sockets.  This allows us to
directly know the side for spliced connections.  For "tap"
connections, it's pretty meaningless, since the side is always the
socket side.  It still makes logical sense though, and it may become
important for future flow table work.

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

passt.h

@@ -37,6 +37,7 @@ union epoll_ref;
 
 #include "pif.h"
 #include "packet.h"
+#include "flow.h"
 #include "icmp.h"
 #include "port_fwd.h"
 #include "tcp.h"
@@ -91,6 +92,7 @@ union epoll_ref {
 		int32_t		fd:FD_REF_BITS;
 		union {
 			uint32_t flow;
+			flow_sidx_t flowside;
 			union tcp_listen_epoll_ref tcp_listen;
 			union udp_epoll_ref udp;
 			union icmp_epoll_ref icmp;