pskb_may_pull() can change skb->head, so we must init iph/greh after
calling it.

Bug added in commit c5441932 (GRE: Refactor GRE tunneling code.)
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>