compat: ip6_gre: fix possible use-after-free in ip6erspan_rcv

compat: ip6_gre: fix possible use-after-free in ip6erspan_rcv

Upstream commit:
    commit 2a3cabae4536edbcb21d344e7aa8be7a584d2afb
    Author: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
    Date:   Sat Apr 6 17:16:53 2019 +0200

    net: ip6_gre: fix possible use-after-free in ip6erspan_rcv

    erspan_v6 tunnels run __iptunnel_pull_header on received skbs to remove
    erspan header. This can determine a possible use-after-free accessing
    pkt_md pointer in ip6erspan_rcv since the packet will be 'uncloned'
    running pskb_expand_head if it is a cloned gso skb (e.g if the packet has
    been sent though a veth device). Fix it resetting pkt_md pointer after
    __iptunnel_pull_header

    Fixes: 1d7e2ed22f8d ("net: erspan: refactor existing erspan code")
    Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Fixes: c387d8177f20 ("compat: Add ipv6 GRE and IPV6 Tunneling")
Cc: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>