Don't attempt atomic route changes under Linux.

The netlink API doesn't natively support atomic route changes.  We used
to attempt to install the new route before removing the old route, but that
would fail in mysterious ways on non-multipath kernels, leading to "stuck"
routes.  Avoid the pain, just take the native approach.

kernel_netlink.c

@@ -864,43 +864,27 @@ kernel_route(int operation, const unsigned char *dest, unsigned short plen,
     ipv4 = v4mapped(gate);
 
     if(operation == ROUTE_MODIFY) {
-        int added;
         if(newmetric == metric && memcmp(newgate, gate, 16) == 0 &&
            newifindex == ifindex)
             return 0;
-        /* It is better to add the new route before removing the old
-           one, to avoid losing packets.  However, if the old and new
-           priorities are equal, this only works if the kernel supports
-           ECMP.  So we first try the "right" order, and fall back on
-           the "wrong" order if it fails with EEXIST. */
+        /* It would be better to add the new route before removing the
+           old one, to avoid losing packets.  However, this causes
+           problems with non-multipath kernels, which sometimes
+           silently fail the request, causing "stuck" routes.  Let's
+           stick with the naive approach, and hope that the window is
+           small enough to be negligible. */
+        kernel_route(ROUTE_FLUSH, dest, plen,
+                     gate, ifindex, metric,
+                     NULL, 0, 0);
         rc = kernel_route(ROUTE_ADD, dest, plen,
                           newgate, newifindex, newmetric,
                           NULL, 0, 0);
         if(rc < 0) {
-            if(errno != EEXIST)
-                return rc;
-            added = 0;
-        } else {
-            added = 1;
+            if(errno == EEXIST)
+                rc = 1;
+            /* Should we try to re-install the flushed route on failure?
+               Error handling is hard. */
         }
-
-        kernel_route(ROUTE_FLUSH, dest, plen,
-                     gate, ifindex, metric,
-                     NULL, 0, 0);
-
-        if(!added) {
-            rc = kernel_route(ROUTE_ADD, dest, plen,
-                              newgate, newifindex, newmetric,
-                              NULL, 0, 0);
-            if(rc < 0) {
-                if(errno == EEXIST)
-                    rc = 1;
-                /* In principle, we should try to re-install the flushed
-                   route on failure to preserve.  However, this should
-                   hopefully not matter much in practice. */
-            }
-        }
-
         return rc;
     }