netfilter: nf_conntrack: remove an atomic bit operation

As this ct won't be seen by the others, we don't need to set the
IPS_CONFIRMED_BIT in atomic way.

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Cc: Tim Gardner <tim.gardner@canonical.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>