mxrouter: add option disable_ipv4_rp_filter for interface

When we have unsymmetrical routing, we need to accept
foreing packets. Make ipv4_rp_filter optional.

mxrouter/mxrouterctl

@@ -350,6 +350,13 @@ sync=1
 __EOF__
 }
 
+our %disable_ipv4_rp_filter = ('all' => 1);  # this is AND ( https://marc.info/?l=linux-kernel&m=123606366021995&w=2 )
+
+sub disable_ipv4_rp_filter {
+	my ($if)=@_;
+	$disable_ipv4_rp_filter{$if}=1;
+}
+
 sub start {
 
 	-d "/var/run/mxrouter/$NETNS" or sys ('mkdir','-p',"/var/run/mxrouter/$NETNS");
@@ -479,9 +486,11 @@ sub start {
 			warn "disable IPV4 send redirects on $dev\n" unless $opt_quiet;
 			set_ipv4_send_redirects($dev,0);
 		}
-		if (get_ipv4_rp_filter($dev)==0) {
-			warn "enable reverse path filter on $dev\n" unless $opt_quiet;
-			set_ipv4_rp_filter($dev,1);
+
+		my $ipv4_rp_filter = $disable_ipv4_rp_filter{$dev} ? 0 : 1;
+		if (get_ipv4_rp_filter($dev)!=$ipv4_rp_filter) {
+			warn (($ipv4_rp_filter ? "enable" : "disable")." ipv4_rp_filter   on $dev\n") unless $opt_quiet;
+			set_ipv4_rp_filter($dev,$ipv4_rp_filter);
 		}
 #		if (get_ipv4_log_martians($dev)==0) {
 #			warn "enable martians log on $dev\n" unless $opt_quiet;