datapath:backport: openvswitch: Add packet truncation support.

datapath:backport: openvswitch: Add packet truncation support.

Upstream commit:
    commit f2a4d086ed4c588d32fe9b7aa67fead7280e7bf1
    Author: William Tu <u9012063@gmail.com>
    Date:   Fri Jun 10 11:49:33 2016 -0700

    openvswitch: Add packet truncation support.

    The patch adds a new OVS action, OVS_ACTION_ATTR_TRUNC, in order to
    truncate packets. A 'max_len' is added for setting up the maximum
    packet size, and a 'cutlen' field is to record the number of bytes
    to trim the packet when the packet is outputting to a port, or when
    the packet is sent to userspace.

    Signed-off-by: William Tu <u9012063@gmail.com>
    Cc: Pravin Shelar <pshelar@nicira.com>
    Acked-by: Pravin B Shelar <pshelar@ovn.org>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: William Tu <u9012063@gmail.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>