netfilter: nf_conntrack: use this_cpu_inc()

this_cpu_inc() is IRQ safe and faster than
local_bh_disable()/__this_cpu_inc()/local_bh_enable(), at least on x86.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Patrick McHardy <kaber@trash.net>
Cc: Christoph Lameter <cl@linux.com>
Cc: Tejun Heo <tj@kernel.org>
Reviewed-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>