make set_blocksize() fail unless block device is opened exclusive

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Documentation/filesystems/porting.rst

@@ -1134,3 +1134,10 @@ superblock of the main block device, i.e., the one stored in sb->s_bdev. Block
 device freezing now works for any block device owned by a given superblock, not
 just the main block device. The get_active_super() helper and bd_fsfreeze_sb
 pointer are gone.
+
+---
+
+**mandatory**
+
+set_blocksize() takes opened struct file instead of struct block_device now
+and it *must* be opened exclusive.

block/bdev.c

@@ -157,6 +157,9 @@ int set_blocksize(struct file *file, int size)
 	if (size < bdev_logical_block_size(bdev))
 		return -EINVAL;
 
+	if (!file->private_data)
+		return -EINVAL;
+
 	/* Don't change the size if it is same as current */
 	if (inode->i_blkbits != blksize_bits(size)) {
 		sync_blockdev(bdev);