netfilter: conntrack: use nf_ct_key_equal() in more places

This prepares for upcoming change that places all conntracks into a
single, global table.  For this to work we will need to also compare
net pointer during lookup.  To avoid open-coding such check use the
nf_ct_key_equal helper and then later extend it to also consider net_eq.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>