netlink: Add common function for extracting payload

Change-Id: I809bd17d50ccb8a8f624b6117c088193f55e79de
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/system_queries.cpp b/src/system_queries.cpp
index aa8e46b..0152f4a 100644
--- a/src/system_queries.cpp
+++ b/src/system_queries.cpp
@@ -131,7 +131,7 @@
     {
         throw std::runtime_error("Not an interface msg");
     }
-    auto ifinfo = stdplus::raw::extract<ifinfomsg>(msg);
+    const auto& ifinfo = netlink::extractRtData<ifinfomsg>(msg);
     InterfaceInfo ret;
     ret.flags = ifinfo.ifi_flags;
     ret.idx = ifinfo.ifi_index;