ip_gre: fix a possible crash in ipgre_err()

Another fix needed in ipgre_err(), as parse_gre_header() might change
skb->head.

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>