nvme: flush reset_work before safely continuing with delete operation

Prevent racing controller reset and delete flows. reset_work must not
ever self-requeue so flushing it suffices.

Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>