iptunnel: scrub packet in iptunnel_pull_header

Part of skb_scrub_packet was open coded in iptunnel_pull_header. Let it call
skb_scrub_packet directly instead.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>