netfilter: conntrack: adjust nf_conntrack_buckets default value

Manually bumping either nf_conntrack_buckets or nf_conntrack_max has
become a common task as our Linux servers tend to serve more and more
clients/applications, so let's adjust nf_conntrack_buckets this to a
more updated value.

Now for systems with more than 4GB of memory, nf_conntrack_buckets
becomes 65536 instead of 16384, resulting in nf_conntrack_max=256k
entries.

Signed-off-by: Marcelo Ricardo Leitner <mleitner@redhat.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>