net: erspan: reload pointer after pskb_may_pull

pskb_may_pull() can change skb->data, so we need to re-load pkt_md
and ershdr at the right place.

Fixes: 94d7d8f2 ("ip6_gre: add erspan v2 support")
Fixes: f551c91d ("net: erspan: introduce erspan v2 for ip_gre")
Signed-off-by: William Tu <u9012063@gmail.com>
Cc: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>