netfilter: cttimeout: fix dependency with l4protocol conntrack module

This patch introduces nf_conntrack_l4proto_find_get() and
nf_conntrack_l4proto_put() to fix module dependencies between
timeout objects and l4-protocol conntrack modules.

Thus, we make sure that the module cannot be removed if it is
used by any of the cttimeout objects.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>