block: revert block_dev read-only check

This reverts commit 75f1dc0d ("block: check bdev_read_only() from
blkdev_get()").  That commit added stricter checking to make sure
devices that were being used read-only were actually opened in that
mode.

It turns out that the change breaks a bunch of kernel code that opens
block devices.  Affected systems include dm, md, and the loop device.
Because strict checking for read-only opens of block devices was not
done before this, the code that opens the devices was opening them
read-write even if they were being used read-only.  Auditing all that
code will take time, and new userspace packages for dm, mdadm, etc.
will also be required.
Signed-off-by: Chuck Ebbert <cebbert@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/block_dev.c

@@ -1215,12 +1215,6 @@ int blkdev_get(struct block_device *bdev, fmode_t mode, void *holder)
 
 	res = __blkdev_get(bdev, mode, 0);
 
-	/* __blkdev_get() may alter read only status, check it afterwards */
-	if (!res && (mode & FMODE_WRITE) && bdev_read_only(bdev)) {
-		__blkdev_put(bdev, mode, 0);
-		res = -EACCES;
-	}
-
 	if (whole) {
 		/* finish claiming */
 		mutex_lock(&bdev->bd_mutex);
@@ -1298,6 +1292,11 @@ struct block_device *blkdev_get_by_path(const char *path, fmode_t mode,
 	if (err)
 		return ERR_PTR(err);
 
+	if ((mode & FMODE_WRITE) && bdev_read_only(bdev)) {
+		blkdev_put(bdev, mode);
+		return ERR_PTR(-EACCES);
+	}
+
 	return bdev;
 }
 EXPORT_SYMBOL(blkdev_get_by_path);