net: sched: act_ctinfo: fix policy validation

Fix nla_policy definition by specifying an exact length type attribute
to CTINFO action paraneter block structure.  Without this change,
netlink parsing will fail validation and the action will not be
instantiated.

8cb08174 ("netlink: make validation more configurable for future")
introduced much stricter checking to attributes being passed via
netlink.  Existing actions were updated to use less restrictive
deprecated versions of nla_parse_nested.

As a new module, act_ctinfo should be designed to use the strict
checking model otherwise, well, what was the point of implementing it.

Confession time: Until very recently, development of this module has
been done on 'net-next' tree to 'clean compile' level with run-time
testing on backports to 4.14 & 4.19 kernels under openwrt.  This is how
I managed to miss the run-time impacts of the new strict
nla_parse_nested function.  I hopefully have learned something from this
(glances toward laptop running a net-next kernel)

There is however a still outstanding implication on iproute2 user space
in that it needs to be told to pass nested netlink messages with the
nested attribute actually set.  So even with this kernel fix to do
things correctly you still cannot instantiate a new 'strict'
nla_parse_nested based action such as act_ctinfo with iproute2's tc.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>