net: ip_gre: fix possible use-after-free in erspan_rcv

erspan tunnels run __iptunnel_pull_header on received skbs to remove
gre and erspan headers. This can determine a possible use-after-free
accessing pkt_md pointer in erspan_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: 1d7e2ed2

 ("net: erspan: refactor existing erspan code")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>