increase default nf_conntrack_max to kernel default

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.

[0]: https://www.kernel.org/doc/Documentation/networking/nf_conntrack-sysctl.txt

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>