zebra: implement per-route mtu handling

This commits allow overriding MTU using netlink attributes on
per-route basis. This is useful for routing protocols that can
advertice prefix specific MTUs between routers (e.g. NHRP).

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
diff --git a/zebra/zebra_vty.c b/zebra/zebra_vty.c
index 6ad286f..b37b901 100644
--- a/zebra/zebra_vty.c
+++ b/zebra/zebra_vty.c
@@ -1290,6 +1290,8 @@
               VTY_NEWLINE);
       vty_out (vty, "  Known via \"%s\"", zebra_route_string (rib->type));
       vty_out (vty, ", distance %u, metric %u", rib->distance, rib->metric);
+      if (rib->mtu)
+        vty_out (vty, ", mtu %u", rib->mtu);
       vty_out (vty, ", vrf %u", rib->vrf_id);
       if (CHECK_FLAG (rib->flags, ZEBRA_FLAG_SELECTED))
         vty_out (vty, ", best");