Add a new OVS action check_pkt_larger

Add a new OVS action check_pkt_larger

This patch adds a new action 'check_pkt_larger' which checks if the
packet is larger than the given size and stores the result in the
destination register.

Usage: check_pkt_larger(len)->REGISTER
Eg. match=...,actions=check_pkt_larger(1442)->NXM_NX_REG0[0],next;

This patch makes use of the new datapath action - 'check_pkt_len'
which was recently added in the commit [1].
At the start of ovs-vswitchd, datapath is probed for this action.
If the datapath action is present, then 'check_pkt_larger'
makes use of this datapath action.

Datapath action 'check_pkt_len' takes these nlattrs
      * OVS_CHECK_PKT_LEN_ATTR_PKT_LEN - 'pkt_len' to check for
      * OVS_CHECK_PKT_LEN_ATTR_ACTIONS_IF_GREATER (optional) - Nested actions
        to apply if the packet length is greater than the specified 'pkt_len'
      * OVS_CHECK_PKT_LEN_ATTR_ACTIONS_IF_LESS_EQUAL (optional) - Nested
        actions to apply if the packet length is lesser or equal to the
        specified 'pkt_len'.

Let's say we have these flows added to an OVS bridge br-int

table=0, priority=100 in_port=1,ip,actions=check_pkt_larger:100->NXM_NX_REG0[0],resubmit(,1)
table=1, priority=200,in_port=1,ip,reg0=0x1/0x1 actions=output:3
table=1, priority=100,in_port=1,ip,actions=output:4

Then the action 'check_pkt_larger' will be translated as
  - check_pkt_len(size=100,gt(3),le(4))

datapath will check the packet length and if the packet length is greater than 100,
it will output to port 3, else it will output to port 4.

In case, datapath doesn't support 'check_pkt_len' action, the OVS action
'check_pkt_larger' sets SLOW_ACTION so that datapath flow is not added.

This OVS action is intended to be used by OVN to check the packet length
and generate an ICMP packet with type 3, code 4 and next hop mtu
in the logical router pipeline if the MTU of the physical interface
is lesser than the packet length. More information can be found here [2]

[1] - https://kernel.googlesource.com/pub/scm/linux/kernel/git/davem/net-next/+/4d5ec89fc8d14dcdab7214a0c13a1c7321dc6ea9
[2] - https://mail.openvswitch.org/pipermail/ovs-discuss/2018-July/047039.html

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: Ben Pfaff <blp@ovn.org>
CC: Gregory Rose <gvrose8192@gmail.com>
Acked-by: Mark Michelson <mmichels@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>