[NETFILTER] ip_conntrack: Make "hashsize" conntrack parameter writable

It's fairly simple to resize the hash table, but currently you need to
remove and reinsert the module.  That's bad (we lose connection
state).  Harald has even offered to write a daemon which sets this
based on load.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>