nvme: Fix NULL dereference on reservation request

This fixes using the NULL 'head' before getting the reference. It is
however possible the head will always be NULL, so this patch uses the
struct nvme_ns to get the ns_id field.

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