Pull in upstream kernel_setup_interface

This pulls the latest version of the kernel_setup_interface function
in from upstream with the hope that it fixes some obscure issues we're
having.

setsockopt(IPV6_JOIN_GROUP): Out of memory
setsockopt(IPV6_LEAVE_GROUP): Address not available
Warning: cannot restore old configuration for wgA.
Warning: cannot save old configuration for wgB.

We keep seeing these sorts of error messages on long running production
nodes, presumably due to the race condition outlined here

sudomesh/bugs#24

Obviously it would be best if we could recover from these errors in
Babel rather than having to try and reduce them on the side of the
interfacing application.

That being said this isn't a well consdiered change, it may be that we
have to cleanup old_if in this error case in a way upstream has not
considered.

kernel_netlink.c

@@ -659,6 +659,7 @@ get_old_if(const char *ifname)
 int
 kernel_setup_interface(int setup, const char *ifname, int ifindex)
 {
+
     char buf[100];
     int i, rc;
 
@@ -677,14 +678,18 @@ kernel_setup_interface(int setup, const char *ifname, int ifindex)
             fprintf(stderr,
                     "Warning: cannot save old configuration for %s.\n",
                     ifname);
-        rc = write_proc(buf, 0);
-        if(rc < 0)
-            return -1;
+        if(old_if[i].rp_filter) {
+            rc = write_proc(buf, 0);
+            if(rc < 0)
+                return -1;
+        }
     } else {
-        if(i >= 0 && old_if[i].rp_filter >= 0)
+        if(i >= 0 && old_if[i].rp_filter > 0)
             rc = write_proc(buf, old_if[i].rp_filter);
-        else
+        else if(i < 0)
             rc = -1;
+        else
+            rc = 1;
 
         if(rc < 0)
             fprintf(stderr,