netfilter: conntrack: convert sysctls to u8

log_invalid sysctl allows values of 0 to 255 inclusive so we no longer
need a range check: the min/max values can be removed.

This also removes all member variables that were moved to net_generic
data in previous patches.

This reduces size of netns_ct struct by one cache line.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>