harness: Fix PROT_WRITE mmap check
    
    This partially reverts commit d7f5065448efb49b2a26e728ff735e12ea05b62e.
    
    The actual problem in the original code was that read() was being used
    to assert whether the buffer was readable, but the kernel was instead
    reading from the file descriptor and then writing into the buffer, so
    no EFAULT was being generated (on architectures that do so).
    
    We needed to use a write() so that the kernel would read from the
    buffer.
    
    Signed-off-by: Guillem Jover <guillem@hadrons.org>
    Signed-off-by: Jeff Moyer <jmoyer@redhat.com>