NVMe: Namespace IDs are unsigned

The 'Number of Namespaces' read from the device was being treated as
signed, which would cause us to not scan any namespaces for a device
with more than 2 billion namespaces.  That led to noticing that the
namespace ID was also being treated as signed, which could lead to the
result from NVME_IOCTL_ID being treated as an error code.

Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>

include/linux/nvme.h

@@ -104,7 +104,7 @@ struct nvme_ns {
 	struct request_queue *queue;
 	struct gendisk *disk;
 
-	int ns_id;
+	unsigned ns_id;
 	int lba_shift;
 	int ms;
 	u64 mode_select_num_blocks;