nvme: fix per-namespace chardev deletion

Decrease reference count of chardevice during char device deletion in
order to fix a memory leak.  Add a release callabck for the device
associated chardev and move ida_simple_remove into the release function.

Fixes: 2637baed

 ("nvme: introduce generic per-namespace chardev")
Reported-by: Yi Zhang <yi.zhang@redhat.com>
Suggested-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Adam Manzanares <a.manzanares@samsung.com>
Reviewed-by: Javier González <javier@javigon.com>
Tested-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>