ipv4: Kill routes during PMTU/redirect updates.

Mark them obsolete so there will be a re-lookup to fetch the
FIB nexthop exception info.

Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/dst.h

@@ -78,6 +78,7 @@ struct dst_entry {
 #define DST_OBSOLETE_NONE	0
 #define DST_OBSOLETE_DEAD	2
 #define DST_OBSOLETE_FORCE_CHK	-1
+#define DST_OBSOLETE_KILL	-2
 	unsigned short		header_len;	/* more space at head required */
 	unsigned short		trailer_len;	/* space to reserve at tail */
 #ifdef CONFIG_IP_ROUTE_CLASSID