[NETFILTER]: fix type of sysctl variables in nf_conntrack_ipv6

These variables should be unsigned.  This fixes sysctl handler for
nf_ct_frag6_{low,high}_thresh.

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>