[IPV4]: Primary and secondary addresses

Add an option to make secondary IP addresses get promoted
when primary IP addresses are removed from the device.
It defaults to off to preserve existing behavior.

Signed-off-by: Harald Welte <laforge@gnumonks.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/sysctl.h

@@ -399,6 +399,7 @@ enum
 	NET_IPV4_CONF_FORCE_IGMP_VERSION=17,
 	NET_IPV4_CONF_ARP_ANNOUNCE=18,
 	NET_IPV4_CONF_ARP_IGNORE=19,
+	NET_IPV4_CONF_PROMOTE_SECONDARIES=20,
 	__NET_IPV4_CONF_MAX
 };