nvme: move namespace scanning to core

Move the scan work item and surrounding code to the common code.  For now
we need a new finish_scan method to allow the PCI driver to set the
irq affinity hints, but I have plans in the works to obsolete this as well.

Note that this moves the namespace scanning from nvme_wq to the system
workqueue, but as we don't rely on namespace scanning to finish from reset
or I/O this should be fine.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by Jon Derrick: <jonathan.derrick@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>