block: keep bdi->io_pages in sync with max_sectors_kb for stacked devices

Field bdi->io_pages added in commit 9491ae4aad ("mm: don't cap request
size based on read-ahead setting") removes unneeded split of read requests.

Stacked drivers do not call blk_queue_max_hw_sectors(). Instead they set
limits of their devices by blk_set_stacking_limits() + disk_stack_limits().
Field bio->io_pages stays zero until user set max_sectors_kb via sysfs.

This patch updates io_pages after merging limits in disk_stack_limits().

Commit c6d6e9b0f6 ("dm: do not allow readahead to limit IO size") fixed
the same problem for device-mapper devices, this one fixes MD RAIDs.

Fixes: 9491ae4aad ("mm: don't cap request size based on read-ahead setting")
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Song Liu <songliubraving@fb.com>

block/blk-settings.c

@@ -664,6 +664,9 @@ void disk_stack_limits(struct gendisk *disk, struct block_device *bdev,
 		printk(KERN_NOTICE "%s: Warning: Device %s is misaligned\n",
 		       top, bottom);
 	}
+
+	t->backing_dev_info->io_pages =
+		t->limits.max_sectors >> (PAGE_SHIFT - 9);
 }
 EXPORT_SYMBOL(disk_stack_limits);