datapath: compat: Backport nf_conntrack_timeout support

datapath: compat: Backport nf_conntrack_timeout support

This patch brings in nf_ct_timeout_put() and nf_ct_set_timeout()
when it is not available in the kernel.

Three symbols are created in acinclude.m4.

* HAVE_NF_CT_SET_TIMEOUT is used to determine if upstream net-next commit
717700d183d65 ("netfilter: Export nf_ct_{set,destroy}_timeout()") is
availabe.  If it is defined, the kernel should have all the
nf_conntrack_timeout support that OVS needs.

* HAVE_NF_CT_TIMEOUT is used to check if upstream net-next commit
6c1fd7dc489d9 ("netfilter: cttimeout: decouple timeout policy from
nfnetlink_cttimeout object") is there.  If it is not defined, we
will use the old ctnl_timeout interface rather than the nf_ct_timeout
interface that is introduced in this commit.

* HAVE_NF_CT_TIMEOUT_FIND_GET_HOOK_NET is used to check if upstream
commit 19576c9478682 ("netfilter: cttimeout: add netns support") is
there, so that we pass different arguement based on whether the kernel
has netns support.

Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Justin Pettit <jpettit@ovn.org>