common: OpTracker doesn't visit TrackedOp when nref == 0.

common: OpTracker doesn't visit TrackedOp when nref == 0.

The patch fixes a race condition that happens between
`unregister_inflight_op` and `visit_ops_in_flight` of
`OpTracker`. When a callable passed to the former one
turns the plain reference it gets into `TrackedOpRef`,
an almost-to-terminate `TrackedOp` (with `nref == 0`)
can be resurrected (`nref++`). This will be reflected
in extra call to `unregister_inflight_op` for same op
leading to e.g. use-after-free. For more details see:
https://tracker.ceph.com/issues/24037#note-5.

The fix deals with the problem by ensuring there will
be no call to the visitor for ops with zeroized `nref`.

Fixes: http://tracker.ceph.com/issues/24037
Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>