overlay/06[89]: fix test run with nfs_export feature enabled by default

overlay/06[89]: fix test run with nfs_export feature enabled by default

The tests were checking that nfs_export feature was successfully enabled
by greping for nfs_export=on option in /proc/mounts.

This check was incorrect if the module default was nfs_export enabled
and caused test to not run with the message:
    cannot enable nfs_export feature on nested overlay

Use a helper that checks this condition correctly.

Reported-by: Chengguang Xu <cgxu519@mykernel.net>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>