block: increment diskseq on all media change events

Currently, associating a loop device with a different file descriptor
does not increment its diskseq.  This allows the following race
condition:

1. Program X opens a loop device
2. Program X gets the diskseq of the loop device.
3. Program X associates a file with the loop device.
4. Program X passes the loop device major, minor, and diskseq to
   something.
5. Program X exits.
6. Program Y detaches the file from the loop device.
7. Program Y attaches a different file to the loop device.
8. The opener finally gets around to opening the loop device and checks
   that the diskseq is what it expects it to be.  Even though the
   diskseq is the expected value, the result is that the opener is
   accessing the wrong file.

From discussions with Christoph Hellwig, it appears that
disk_force_media_change() was supposed to call inc_diskseq(), but in
fact it does not.  Adding a Fixes: tag to indicate this.  Christoph's
Reported-by is because he stated that disk_force_media_change()
calls inc_diskseq(), which is what led me to discover that it should but
does not.

Reported-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Fixes: e6138dc12d ("block: add a helper to raise a media changed event")
Cc: stable@vger.kernel.org # 5.15+
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230607170837.1559-1-demi@invisiblethingslab.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/disk-events.c

@@ -307,6 +307,7 @@ bool disk_force_media_change(struct gendisk *disk, unsigned int events)
 	if (!(events & DISK_EVENT_MEDIA_CHANGE))
 		return false;
 
+	inc_diskseq(disk);
 	if (__invalidate_device(disk->part0, true))
 		pr_warn("VFS: busy inodes on changed media %s\n",
 			disk->disk_name);