net: openvswitch: Add a new action check_pkt_len

This patch adds a new action - 'check_pkt_len' which checks the
packet length and executes a set of actions if the packet
length is greater than the specified length or executes
another set of actions if the packet length is lesser or equal to.

This action takes below nlattrs
  * OVS_CHECK_PKT_LEN_ATTR_PKT_LEN - 'pkt_len' to check for

  * OVS_CHECK_PKT_LEN_ATTR_ACTIONS_IF_GREATER - Nested actions
    to apply if the packet length is greater than the specified 'pkt_len'

  * OVS_CHECK_PKT_LEN_ATTR_ACTIONS_IF_LESS_EQUAL - Nested
    actions to apply if the packet length is lesser or equal to the
    specified 'pkt_len'.

The main use case for adding this action is to solve the packet
drops because of MTU mismatch in OVN virtual networking solution.
When a VM (which belongs to a logical switch of OVN) sends a packet
destined to go via the gateway router and if the nic which provides
external connectivity, has a lesser MTU, OVS drops the packet
if the packet length is greater than this MTU.

With the help of this action, OVN will check the packet length
and if it is greater than the MTU size, it will generate an
ICMP packet (type 3, code 4) and includes the next hop mtu in it
so that the sender can fragment the packets.

Reported-at:
https://mail.openvswitch.org/pipermail/ovs-discuss/2018-July/047039.html
Suggested-by: Ben Pfaff <blp@ovn.org>
Signed-off-by: Numan Siddique <nusiddiq@redhat.com>
CC: Gregory Rose <gvrose8192@gmail.com>
CC: Pravin B Shelar <pshelar@ovn.org>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Tested-by: Greg Rose <gvrose8192@gmail.com>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>