fetch2: Ensure directory exists before creating symlink
    
    If the mirrors code is trying to create a symlink and the
    parent directory doesn't exist, as might be the case for sstate
    mirrors where the fetch is into a subdir, it can silently fail.
    
    Ensure the directory exists in this case to avoid issues.
    
    Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>