NVMe: Namespace removal simplifications

This liberates namespace removal from the device, allowing gendisk
references to be closed independent of the nvme controller reference
count.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>