btrfs: s_bdev is not null after missing replace

Yauhen reported in the ML that s_bdev is null at mount, and
s_bdev gets updated to some device when missing device is
replaced, as because bdev is null for missing device, things
gets matched up. Fix this by checking if s_bdev is set. I
didn't want to completely remove updating s_bdev because
the future multi device support at vfs layer may need it.
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reported-by: Yauhen Kharuzhy <yauhen.kharuzhy@zavadatar.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/dev-replace.c

@@ -569,7 +569,8 @@ static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,
 	ASSERT(list_empty(&src_device->resized_list));
 	tgt_device->commit_total_bytes = src_device->commit_total_bytes;
 	tgt_device->commit_bytes_used = src_device->bytes_used;
-	if (fs_info->sb->s_bdev == src_device->bdev)
+	if (fs_info->sb->s_bdev &&
+		(fs_info->sb->s_bdev == src_device->bdev))
 		fs_info->sb->s_bdev = tgt_device->bdev;
 	if (fs_info->fs_devices->latest_bdev == src_device->bdev)
 		fs_info->fs_devices->latest_bdev = tgt_device->bdev;

fs/btrfs/volumes.c

@@ -1848,7 +1848,8 @@ int btrfs_rm_device(struct btrfs_root *root, char *device_path, u64 devid)
 
 	next_device = list_entry(root->fs_info->fs_devices->devices.next,
 				 struct btrfs_device, dev_list);
-	if (device->bdev == root->fs_info->sb->s_bdev)
+	if (root->fs_info->sb->s_bdev &&
+		(root->fs_info->sb->s_bdev == device->bdev))
 		root->fs_info->sb->s_bdev = next_device->bdev;
 	if (device->bdev == root->fs_info->fs_devices->latest_bdev)
 		root->fs_info->fs_devices->latest_bdev = next_device->bdev;
@@ -1996,7 +1997,8 @@ void btrfs_destroy_dev_replace_tgtdev(struct btrfs_fs_info *fs_info,
 
 	next_device = list_entry(fs_info->fs_devices->devices.next,
 				 struct btrfs_device, dev_list);
-	if (tgtdev->bdev == fs_info->sb->s_bdev)
+	if (fs_info->sb->s_bdev &&
+		(tgtdev->bdev == fs_info->sb->s_bdev))
 		fs_info->sb->s_bdev = next_device->bdev;
 	if (tgtdev->bdev == fs_info->fs_devices->latest_bdev)
 		fs_info->fs_devices->latest_bdev = next_device->bdev;