doc: fix link to MSG_ZEROCOPY patchset

Use https and link to the patch directly.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/msg_zerocopy.rst

@@ -50,7 +50,7 @@ the excellent reporting over at LWN.net or read the original code.
 
   patchset
     [PATCH net-next v4 0/9] socket sendmsg MSG_ZEROCOPY
-    http://lkml.kernel.org/r/20170803202945.70750-1-willemdebruijn.kernel@gmail.com
+    https://lkml.kernel.org/netdev/20170803202945.70750-1-willemdebruijn.kernel@gmail.com
 
 
 Interface