ofp-actions: Add truncate action.

ofp-actions: Add truncate action.

The patch adds a new action to support packet truncation.  The new action
is formatted as 'output(port=n,max_len=m)', as output to port n, with
packet size being MIN(original_size, m).

One use case is to enable port mirroring to send smaller packets to the
destination port so that only useful packet information is mirrored/copied,
saving some performance overhead of copying entire packet payload.  Example
use case is below as well as shown in the testcases:

    - Output to port 1 with max_len 100 bytes.
    - The output packet size on port 1 will be MIN(original_packet_size, 100).
    # ovs-ofctl add-flow br0 'actions=output(port=1,max_len=100)'

    - The scope of max_len is limited to output action itself.  The following
      packet size of output:1 and output:2 will be intact.
    # ovs-ofctl add-flow br0 \
            'actions=output(port=1,max_len=100),output:1,output:2'
    - The Datapath actions shows:
    # Datapath actions: trunc(100),1,1,2

Tested-at: https://travis-ci.org/williamtu/ovs-travis/builds/140037134
Signed-off-by: William Tu <u9012063@gmail.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>