[NETFILTER]: nf_conntrack: sparse warnings

The hashtable size is really unsigned so sparse complains when you pass
a signed integer.  Change all uses to make it consistent.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>