md/bitmap: don't pass -1 to bitmap_storage_alloc.

Passing -1 to bitmap_storage_alloc() causes page->index to be set to
-1, which is quite problematic.

So only pass ->cluster_slot if mddev_is_clustered().

Fixes: b97e92574c0b ("Use separate bitmaps for each nodes in the cluster")
Cc: stable@vger.kernel.org (v4.1+)
Signed-off-by: NeilBrown <neilb@suse.com>

drivers/md/bitmap.c

@@ -1997,7 +1997,8 @@ int bitmap_resize(struct bitmap *bitmap, sector_t blocks,
 	if (bitmap->mddev->bitmap_info.offset || bitmap->mddev->bitmap_info.file)
 		ret = bitmap_storage_alloc(&store, chunks,
 					   !bitmap->mddev->bitmap_info.external,
-					   bitmap->cluster_slot);
+					   mddev_is_clustered(bitmap->mddev)
+					   ? bitmap->cluster_slot : 0);
 	if (ret)
 		goto err;