netfilter: conntrack: use shared sysctl constants

Use shared sysctl variables for zero and one constants, as in commit
eec4844f ("proc/sysctl: add shared variables for range check")

Fixes: 8f14c99c ("netfilter: conntrack: limit sysctl setting for boolean options")
Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>