block: Fix null_blk_zoned creation failure with small number of zones

null_blk_zoned creation fails if the number of zones specified is equal to or is
smaller than 64 due to a memory allocation failure in blk_alloc_zones(). With
such a small number of zones, the required memory size for all zones descriptors
fits in a single page, and the page order for alloc_pages_node() is zero. Allow
this value in blk_alloc_zones() for the allocation to succeed.

Fixes: bf505456 "block: Introduce blk_revalidate_disk_zones()"
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-zoned.c

@@ -378,7 +378,7 @@ static struct blk_zone *blk_alloc_zones(int node, unsigned int *nr_zones)
 	struct page *page;
 	int order;
 
-	for (order = get_order(size); order > 0; order--) {
+	for (order = get_order(size); order >= 0; order--) {
 		page = alloc_pages_node(node, GFP_NOIO | __GFP_ZERO, order);
 		if (page) {
 			*nr_zones = min_t(unsigned int, *nr_zones,