increase default nf_conntrack_max to kernel default
for nf_conntrack_max the kernel uses by default the value:
(nf_conntrack_buckets value * 4) and nf_conntrack_buckets
is set to 2^16 for machines with more than 4GB memory, so the
resulting default would be 2^18 == 262144.
As PVE hoists are expected to have more than such a, nowadays rather
small, amount of memory, update the default to match the one which
would be normally used anyway.