netfilter: nf_conntrack: fix a race in __nf_conntrack_confirm against nf_ct_get_next_corpse()

This race was triggered by a 'conntrack -F' command running in parallel
to the insertion of a hash for a new connection. Losing this race led to
a dead conntrack entry effectively blocking traffic for a particular
connection until timeout or flushing the conntrack hashes again.
Now the check for an already dying connection is done inside the lock.

Signed-off-by: Joerg Marx <joerg.marx@secunet.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>