nvmet: allow bdev in buffered_io mode

Allow block device to be configured in the buffered I/O mode by using
the file backend. In this way now we can use cache for the block
device namespace which shows significant performance improvement.

We update the block device ns enable function and return early when
buffered_io flag is set.
Signed-off-by: NChaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>

drivers/nvme/target/io-cmd-bdev.c

@@ -76,6 +76,14 @@ int nvmet_bdev_ns_enable(struct nvmet_ns *ns)
 {
 	int ret;
 
+	/*
+	 * When buffered_io namespace attribute is enabled that means user want
+	 * this block device to be used as a file, so block device can take
+	 * an advantage of cache.
+	 */
+	if (ns->buffered_io)
+		return -ENOTBLK;
+
 	ns->bdev = blkdev_get_by_path(ns->device_path,
 			FMODE_READ | FMODE_WRITE, NULL);
 	if (IS_ERR(ns->bdev)) {