netfilter: conntrack: keep BH enabled during lookup

No need to disable BH here anymore:

stats are switched to _ATOMIC variant (== this_cpu_inc()), which
nowadays generates same code as the non _ATOMIC NF_STAT, at least on x86.

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