[XFRM_TUNNEL]: Reload header pointer after pskb_may_pull/pskb_expand_head

Please consider applying, this was found on your latest
net-2.6 tree while playing around with that ip_hdr() + turn
skb->nh/h/mac pointers  as offsets on 64 bits idea :-)
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>