net: ipv4: add ignore_routes_with_linkdown sysctl

Looks like this is new as of the 4.2 timeframe. It does bring up a
question: should we do a ls /proc/sys/net/conf/lo on the host to figure out
what sysctls to try and dump instead of use this hardcoded list? I guess we
need to check file modes too, because of mc_forwarding, but something like
that.

v2: add ignore_routes_with_linkdown to the end of the hardcoded devconfs
    list, so as not to break migration across criu versions.
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

net.c

@@ -85,6 +85,7 @@ static char *devconfs[] = {
 	"shared_media",
 	"src_valid_mark",
 	"tag",
+	"ignore_routes_with_linkdown",
 };
 
 /*

test/zdtm/live/static/netns-dev.c

@@ -25,6 +25,7 @@ char *devconfs[] = {
 	"forwarding",
 	"igmpv2_unsolicited_report_interval",
 	"igmpv3_unsolicited_report_interval",
+	"ignore_routes_with_linkdown",
 	"log_martians",
 	"mc_forwarding",
 	"medium_id",
@@ -57,6 +58,7 @@ int rand_limit[] = {
 	2,	/* forwarding */
 	0,	/* igmpv2_unsolicited_report_interval */
 	0,	/* igmpv3_unsolicited_report_interval */
+	2,	/* ignore_routes_with_linkdown */
 	2,	/* log_martians */
 	2,	/* mc_forwarding */
 	0,	/* medium_id */