Fix btrfs_open_devices to deal with changes since the scan ioctls

Devices can change after the scan ioctls are done, and btrfs_open_devices
needs to be able to verify them as they are opened and used by the FS.

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 38b0d9e..264f297 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1266,10 +1266,10 @@
 
 	btrfs_parse_options(options, tree_root, NULL);
 
-	if (btrfs_super_num_devices(disk_super) > fs_devices->num_devices) {
+	if (btrfs_super_num_devices(disk_super) > fs_devices->open_devices) {
 		printk("Btrfs: wanted %llu devices, but found %llu\n",
 		       (unsigned long long)btrfs_super_num_devices(disk_super),
-		       (unsigned long long)fs_devices->num_devices);
+		       (unsigned long long)fs_devices->open_devices);
 		if (btrfs_test_opt(tree_root, DEGRADED))
 			printk("continuing in degraded mode\n");
 		else {