ip6_gre: simplify gre header parsing in ip6gre_err

Same as ip_gre, use gre_parse_header to parse gre header in gre error
handler code.

Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>