ipv4: avoid a test in ip_rt_put()

We can save a test in ip_rt_put(), considering dst_release() accepts
a NULL parameter, and dst is first element in rtable.

Add a BUILD_BUG_ON() to catch any change that could break this
assertion.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Cong Wang <amwang@redhat.com>
Acked-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/route.h

@@ -198,9 +198,12 @@ struct in_ifaddr;
 extern void fib_add_ifaddr(struct in_ifaddr *);
 extern void fib_del_ifaddr(struct in_ifaddr *, struct in_ifaddr *);
 
-static inline void ip_rt_put(struct rtable * rt)
+static inline void ip_rt_put(struct rtable *rt)
 {
-	if (rt)
+	/* dst_release() accepts a NULL parameter.
+	 * We rely on dst being first structure in struct rtable
+	 */
+	BUILD_BUG_ON(offsetof(struct rtable, dst) != 0);
 	dst_release(&rt->dst);
 }