networking: use skb_put_zero()

Use the recently introduced helper to replace the pattern of
skb_put() && memset(), this transformation was done with the
following spatch:

@@
identifier p;
expression len;
expression skb;
@@
-p = skb_put(skb, len);
-memset(p, 0, len);
+p = skb_put_zero(skb, len);

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>