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

If an origin target has no snapshots, o->split_boundary is set to 0.
This causes BUG_ON(sectors <= 0) in block/bio.c:bio_split().

Fix this by initializing chunk_size, and in turn split_boundary, to
rounddown_pow_of_two(UINT_MAX) -- the largest power of two that fits
into "unsigned" type.
Reported-by: Michael Tokarev <mjt@tls.msk.ru>
Tested-by: Michael Tokarev <mjt@tls.msk.ru>
Cc: stable@vger.kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-snap.c

@@ -855,12 +855,11 @@ 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 = 0;
+	unsigned chunk_size = rounddown_pow_of_two(UINT_MAX);
 
 	if (o)
 		list_for_each_entry(snap, &o->snapshots, list)
-			chunk_size = min_not_zero(chunk_size,
-						  snap->store->chunk_size);
+			chunk_size = min(chunk_size, snap->store->chunk_size);
 
 	return (uint32_t) chunk_size;
 }