nvme: support ranged discard requests

NVMe supports up to 256 ranges per DSM command, so wire up support
for ranged discards up to that limit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

include/linux/nvme.h

@@ -553,6 +553,8 @@ enum {
 	NVME_DSMGMT_AD		= 1 << 2,
 };
 
+#define NVME_DSM_MAX_RANGES	256
+
 struct nvme_dsm_range {
 	__le32			cattr;
 	__le32			nlb;