dm snapshot: revert "fix a crash when an origin has no snapshots"

Commit 7ee06dd ("dm snapshot: fix a
crash when an origin has no snapshots") introduced a regression in
snapshot merging - causing the lvm2 test lvcreate-cache-snapshot.sh
got stuck in an infinite loop.

Even though commit 7ee06dd was marked
for stable@ the stable team was notified to _not_ backport it.

Fixes: 7ee06dd ("dm snapshot: fix a crash when an origin has no snapshots")
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-snap.c

@@ -855,11 +855,12 @@ static int dm_add_exception(void *context, chunk_t old, chunk_t new)
 static uint32_t __minimum_chunk_size(struct origin *o)
 {
 	struct dm_snapshot *snap;
-	unsigned chunk_size = rounddown_pow_of_two(UINT_MAX);
+	unsigned chunk_size = 0;
 
 	if (o)
 		list_for_each_entry(snap, &o->snapshots, list)
-			chunk_size = min(chunk_size, snap->store->chunk_size);
+			chunk_size = min_not_zero(chunk_size,
+						  snap->store->chunk_size);
 
 	return (uint32_t) chunk_size;
 }