xfrm4: Reload skb header pointers after calling pskb_may_pull.

commit ea673a4d

 upstream.

A call to pskb_may_pull may change the pointers into the packet,
so reload the pointers after the call.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>