ip_gre: fix possible use after free

Once skb_realloc_headroom() is called, tiph might point to freed memory.

Cache tiph->ttl value before the reallocation, to avoid unexpected
behavior.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>