datapath: introduce nf_conntrack_helper_put function

datapath: introduce nf_conntrack_helper_put function

Upstream commit:
    commit d91fc59cd77c719f33eda65c194ad8f95a055190
    Author: Liping Zhang <zlpnobody@gmail.com>
    Date:   Sun May 7 22:01:55 2017 +0800

    netfilter: introduce nf_conntrack_helper_put helper function

    And convert module_put invocation to nf_conntrack_helper_put, this is
    prepared for the followup patch, which will add a refcnt for cthelper,
    so we can reject the deleting request when cthelper is in use.

    Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
    Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Applied with additional use of HAVE_NF_CONNTRACK_HELPER_PUT compatibility
flag defined in acinclude.m4.

Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Joe Stringer <joe@ovn.org>