btrfs: change BUG_ON to assertion in btrfs_read_roots()

There's one caller of btrfs_read_roots() and that already uses the
tree_root pointer, it's pointless to BUG_ON on it. As it's an assumption
of the initialization helpers make it an assert instead.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -2239,7 +2239,7 @@ static int btrfs_read_roots(struct btrfs_fs_info *fs_info)
 	struct btrfs_key location;
 	int ret;
 
-	BUG_ON(!fs_info->tree_root);
+	ASSERT(fs_info->tree_root);
 
 	ret = load_global_roots(tree_root);
 	if (ret)