ip_gre: check packet length and mtu correctly in erspan tx

ip_gre: check packet length and mtu correctly in erspan tx

Upstream commit:
    commit f192970de860d3ab90aa9e2a22853201a57bde78
    Author: William Tu <u9012063@gmail.com>
    Date:   Thu Oct 5 12:07:12 2017 -0700

    ip_gre: check packet length and mtu correctly in erspan tx

    Similarly to early patch for erspan_xmit(), the ARPHDR_ETHER device
    is the length of the whole ether packet.  So skb->len should subtract
    the dev->hard_header_len.

    Fixes: 1a66a836da63 ("gre: add collect_md mode to ERSPAN tunnel")
    Fixes: 84e54fe0a5ea ("gre: introduce native tunnel support for ERSPAN")
    Signed-off-by: William Tu <u9012063@gmail.com>
    Cc: Xin Long <lucien.xin@gmail.com>
    Cc: David Laight <David.Laight@aculab.com>
    Reviewed-by: Xin Long <lucien.xin@gmail.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Cc: William Tu <u9012063@gmail.com>
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: William Tu <u9012063@gmail.com>