vxlan: fix too large pskb_may_pull with remote checksum

vxlan_remcsum is called after iptunnel_pull_header and thus the skb has
vxlan header already pulled. Don't include vxlan header again in the
calculation.

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