nvme: setup streams after initializing namespace head

Fixes a NULL pointer dereference.

Reported-by: Arnav Dawn <a.dawn@samsung.com>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>