net: Move mtu handling down to the protocol depended handlers

We move all mtu handling from dst_mtu() down to the protocol
layer. So each protocol can implement the mtu handling in
a different manner.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/dst.h b/include/net/dst.h
index 666de31..6faec1a 100644
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -205,12 +205,7 @@
 
 static inline u32 dst_mtu(const struct dst_entry *dst)
 {
-	u32 mtu = dst_metric_raw(dst, RTAX_MTU);
-
-	if (!mtu)
-		mtu = dst->ops->mtu(dst);
-
-	return mtu;
+	return dst->ops->mtu(dst);
 }
 
 /* RTT metrics are stored in milliseconds for user ABI, but used as jiffies */