netfilter: nf_conntrack: fix conntrack lookup race

The RCU protected conntrack hash lookup only checks whether the entry
has a refcount of zero to decide whether it is stale. This is not
sufficient, entries are explicitly removed while there is at least
one reference left, possibly more. Explicitly check whether the entry
has been marked as dying to fix this.

Signed-off-by: Patrick McHardy <kaber@trash.net>