netfilter: nft_ct: load both IPv4 and IPv6 conntrack modules for NFPROTO_INET

The ct expression can currently not be used in the inet family since
we don't have a conntrack module for NFPROTO_INET, so
nf_ct_l3proto_try_module_get() fails. Add some manual handling to
load the modules for both NFPROTO_IPV4 and NFPROTO_IPV6 if the
ct expression is used in the inet family.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>