drivers/net : Correct the size argument to kzalloc

lp->rx_skb has type struct sk_buff **, not struct sk_buff *, so the
elements of the array should have pointer type, not structure type.

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

)

// <smpl>
@disable sizeof_type_expr@
type T;
T **x;
@@

  x =
  <+...sizeof(
- T
+ *x
  )...+>
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>