netfilter: nat: use atomic bit op to clear the _SRC_NAT_DONE_BIT

netfilter: nat: use atomic bit op to clear the _SRC_NAT_DONE_BIT

We need to clear the IPS_SRC_NAT_DONE_BIT to indicate that the ct has
been removed from nat_bysource table. But unfortunately, we use the
non-atomic bit operation: "ct->status &= ~IPS_NAT_DONE_MASK". So
there's a race condition that we may clear the _DYING_BIT set by
another CPU unexpectedly.

Since we don't care about the IPS_DST_NAT_DONE_BIT, so just using
clear_bit to clear the IPS_SRC_NAT_DONE_BIT is enough.

Also note, this is the last user which use the non-atomic bit operation
to update the confirmed ct->status.

Reported-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>