overlay: add a test for multiple redirects to the same lower dir

overlay: add a test for multiple redirects to the same lower dir

Multiple redirects to the same lower dir will falsely return the
same st_ino/st_dev for two different upper dirs and will cause
'diff' to falsely report that content of directories is the same
when it is not.

This test checks that overalyfs detects and fails lookup of a
multiply redirected dir.

The check for multiply redirected dir was implemented by kernel
commit titled "ovl: hash directory inodes for fsnotify".

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