bcache: correct flash only vols (check all uuids)

Commit 2831231d ("bcache: reduce cache_set devices iteration by
devices_max_used") adds c->devices_max_used to reduce iteration of
c->uuids elements, this value is updated in bcache_device_attach().

But for flash only volume, when calling flash_devs_run(), the function
bcache_device_attach() is not called yet and c->devices_max_used is not
updated. The unexpected result is, the flash only volume won't be run
by flash_devs_run().

This patch fixes the issue by iterate all c->uuids elements in
flash_devs_run(). c->devices_max_used will be updated properly when
bcache_device_attach() gets called.

[mlyle: commit subject edited for character limit]

Fixes: 2831231d ("bcache: reduce cache_set devices iteration by devices_max_used")
Reported-by: Tang Junhui <tang.junhui@zte.com.cn>
Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Michael Lyle <mlyle@lyle.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/super.c

@@ -1274,7 +1274,7 @@ static int flash_devs_run(struct cache_set *c)
 	struct uuid_entry *u;
 
 	for (u = c->uuids;
-	     u < c->uuids + c->devices_max_used && !ret;
+	     u < c->uuids + c->nr_uuids && !ret;
 	     u++)
 		if (UUID_FLASH_ONLY(u))
 			ret = flash_dev_run(c, u);