NVMe: Reference count open namespaces

NVMe: Reference count open namespaces

Dynamic namespace attachment means the namespace may be removed at any
time, so the namespace reference count can not be tied to the device
reference count. This fixes a NULL dereference if an opened namespace
is detached from a controller.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
(cherry picked from commit 188c3568f814fea965947ed24739987ba9c5a87e)

Orabug: 22620486
Signed-off-by: Jason Luo <zhangqing.luo@oracle.com>