ipv4: fix a potential use after free in ip_tunnel_core.c

pskb_may_pull() maybe change skb->data and make eth pointer oboslete,
so set eth after pskb_may_pull()

Fixes:3d7b46cd

("ip_tunnel: push generic protocol handling to ip_tunnel module")
Cc: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>