erspan: auto detect truncated ipv6 packets.

erspan: auto detect truncated ipv6 packets.

Upstream commit:
    commit d5db21a3e6977dcb42cee3d16cd69901fa66510a
    Author: William Tu <u9012063@gmail.com>
    Date:   Fri May 11 05:49:47 2018 -0700

    erspan: auto detect truncated ipv6 packets.

    Currently the truncated bit is set only when 1) the mirrored packet
    is larger than mtu and 2) the ipv4 packet tot_len is larger than
    the actual skb->len.  This patch adds another case for detecting
    whether ipv6 packet is truncated or not, by checking the ipv6 header
    payload_len and the skb->len.

    Reported-by: Xiaoyan Jin <xiaoyanj@vmware.com>
    Signed-off-by: William Tu <u9012063@gmail.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: William Tu <u9012063@gmail.com>
Cc: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>