nvme: don't schedule multiple resets

The queue_work only fails if the work is pending, but not yet running. If
the work is running, the work item would get requeued, triggering a
double reset. If the first reset fails for any reason, the second
reset triggers:

	WARN_ON(dev->ctrl.state == NVME_CTRL_RESETTING)

Hitting that schedules controller deletion for a second time, which
potentially takes a reference on the device that is being deleted.
If the reset occurs at the same time as a hot removal event, this causes
a double-free.

This patch has the reset helper function check if the work is busy
prior to queueing, and changes all places that schedule resets to use
this function. Since most users don't want to sync with that work, the
"flush_work" is moved to the only caller that wants to sync.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi <Grimberg&lt;sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>