overlay/018: print hardlink content to golden output

overlay/018: print hardlink content to golden output

diff may skip comparing content of files with identical st_ino/st_dev.
Overlayfs stat(2) may return same st_dev/st_ino for hardlink copy ups,
but it does not mean that read(2) will return the same content.

Convert the test to output hardlink files content to golden output
instead of using diff.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>