ovl: fix mount option checks for nfs_export with no upperdir

ovl: fix mount option checks for nfs_export with no upperdir

Without upperdir mount option, there is no index dir and the dependency
checks nfs_export => index for mount options parsing are incorrect.

Allow the combination nfs_export=on,index=off with no upperdir and move
the check for dependency redirect_dir=nofollow for non-upper mount case
to mount options parsing.

Signed-off-by: Amir Goldstein <[email protected]>
Signed-off-by: Miklos Szeredi <[email protected]>