icmp: Store ping socket information in flow table

Currently icmp_id_map[][] stores information about ping sockets in a
bespoke structure.  Move the same information into new types of flow
in the flow table.  To match that change, replace the existing ICMP
timer with a flow-based timer for expiring ping sockets.  This has the
advantage that we only need to scan the active flows, not all possible
ids.

We convert icmp_id_map[][] to point to the flow table entries, rather
than containing its own information.  We do still use that array for
locating the right ping flows, rather than using a "flow native" form
of lookup for the time being.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
[sbrivio: Update id_sock description in comment to icmp_ping_new()]
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

flow.h

@@ -19,6 +19,10 @@ enum flow_type {
 	FLOW_TCP,
 	/* A TCP connection between a host socket and ns socket */
 	FLOW_TCP_SPLICE,
+	/* ICMP echo requests from guest to host and matching replies back */
+	FLOW_PING4,
+	/* ICMPv6 echo requests from guest to host and matching replies back */
+	FLOW_PING6,
 
 	FLOW_NUM_TYPES,
 };