netfilter: conntrack: fix race in __nf_conntrack_confirm against get_next_corpse

netfilter: conntrack: fix race in __nf_conntrack_confirm against get_next_corpse

After removal of the central spinlock nf_conntrack_lock, in
commit ab9997edf3b6a ("netfilter: conntrack: remove central
spinlock nf_conntrack_lock"), it is possible to race against
get_next_corpse().

The race is against the get_next_corpse() cleanup on
the "unconfirmed" list (a per-cpu list with seperate locking),
which set the DYING bit.

Fix this race, in __nf_conntrack_confirm(), by removing the CT
from unconfirmed list before checking the DYING bit.  In case
race occured, re-add the CT to the dying list.

While at this, fix coding style of the comment that has been
updated.

Fixes: ab9997edf3b6a ("netfilter: conntrack: remove central spinlock nf_conntrack_lock")
Reported-by: bill bonaparte <programme110@gmail.com>
Signed-off-by: bill bonaparte <programme110@gmail.com>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>