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: b97e92574c ("Use separate bitmaps for each nodes in the cluster")
Cc: stable@vger.kernel.org (v4.1+)
Signed-off-by: NeilBrown <neilb@suse.com>