nvme-fc: avoid workqueue flush stalls

There's no need to wait for the full nvme_wq, which is now shared,
to flush. flush only the delete_work item.

Signed-off-by: James Smart <james.smart@broadcom.com>
Reviewed-by: Sagi Grimberg <sgi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>