generic/631: remove useless nfs_export=off

generic/631: remove useless nfs_export=off

The nfs_export feature was introduced since kernel v4.16-rc1 after
commit f168f1098dd9038 ("ovl: add support for "nfs_export"
configuration").  On old kernel, this case will result in infinite
loop because overlay mount failed because of unrecognized mount
options nfs_export=off.

In kernel documentation, overlay.rst said "the mount options
index=off,nfs_export=on are conflicting for a read-write mount and
will result in an error.".  So using single index=off is also ok in
here.

Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>