l2tp: constify net_device_ops structures

l2tp: constify net_device_ops structures

Check for net_device_ops structures that are only stored in the netdev_ops
field of a net_device structure.  This field is declared const, so
net_device_ops structures that have this property can be declared as const
also.

The semantic patch that makes this change is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@r disable optional_qualifier@
identifier i;
position p;
@@
static struct net_device_ops i@p = { ... };

@ok@
identifier r.i;
struct net_device e;
position p;
@@
e.netdev_ops = &i@p;

@bad@
position p != {r.p,ok.p};
identifier r.i;
struct net_device_ops e;
@@
e@i@p

@depends on !bad disable optional_qualifier@
identifier r.i;
@@
static
+const
 struct net_device_ops i = { ... };
// </smpl>

The result of size on this file before the change is:
   text	      data     bss     dec         hex	  filename
   3401        931      44    4376        1118	net/l2tp/l2tp_eth.o

and after the change it is:
   text	     data        bss	    dec	    hex	filename
   3993       347         44       4384    1120	net/l2tp/l2tp_eth.o

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>