Btrfs: unplug all devices in the unplug call back

For larger multi-device filesystems, there was logic to limit the
number of devices unplugged to just the page that was sent to our sync_page
function.

But, the code wasn't always unplugging the right device.  Since this was
just an optimization, disable it for now.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/disk-io.c

@@ -1214,7 +1214,7 @@ void btrfs_unplug_io_fn(struct backing_dev_info *bdi, struct page *page)
 	u64 offset;
 
 	/* the generic O_DIRECT read code does this */
-	if (!page) {
+	if (1 || !page) {
 		__unplug_io_fn(bdi, page);
 		return;
 	}