net: core: Support UID-based routing.

This contains the following commits:

1. cc2f522 net: core: Add a UID range to fib rules.
2. d7ed2bd net: core: Use the socket UID in routing lookups.
3. 2f9306a net: core: Add a RTA_UID attribute to routes.
    This is so that userspace can do per-UID route lookups.
4. 8e46efb net: ipv6: Use the UID in IPv6 PMTUD
    IPv4 PMTUD already does this because ipv4_sk_update_pmtu
    uses __build_flow_key, which includes the UID.

Bug: 15413527
Change-Id: Iae3d4ca3979d252b6cec989bdc1a6875f811f03a
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
diff --git a/include/net/ip.h b/include/net/ip.h
index bc43c0f..f4182b1 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -173,6 +173,7 @@
 				/* -1 if not needed */ 
 	int	    bound_dev_if;
 	u8  	    tos;
+	kuid_t	    uid;
 }; 
 
 #define IP_REPLY_ARG_NOSRCCHECK 1