ipvs: dynamically limit the connection hash table

As we allow the hash table to be configured to rows above 2^20,
we should limit it depending on the available memory to some
sane values. Switch to kvmalloc allocation to better select
the needed allocation type.

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>