btrfs: fix uninitialized return value

When allocation fails in btrfs_read_fs_root_no_name, ret is not set
although it is returned, holding a garbage value.

Signed-off-by: David Sterba <dsterba@suse.cz>
Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>