overlay: test concurrent copy up of lower hardlinks

overlay: test concurrent copy up of lower hardlinks

Two tasks make a modification concurrently on two hardlinks of a
large lower inode.  The copy up should be triggered by one of the
tasks and the other should be waiting for copy up to complete.  Both
copy up targets should end up being upper hardlinks and both
metadata changes should be visible in both hardlinks.

With kernel <= v4.12, hardlinks are broken on copy up, meaning that
copy up is performed independetly and the resulting upper copy up
targets each have only one of the the metadata changes visible.

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