ovl: update ctx->pos on impure dir iteration

ovl: update ctx->pos on impure dir iteration

commit b02a16e6413a2f782e542ef60bad9ff6bf212f8a upstream.

This fixes a regression with readdir of impure dir in overlayfs
that is shared to VM via 9p fs.

Reported-by: Miguel Bernal Marin <miguel.bernal.marin@linux.intel.com>
Fixes: 4edb83bb1041 ("ovl: constant d_ino for non-merge dirs")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Tested-by: Miguel Bernal Marin <miguel.bernal.marin@linux.intel.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>