ovl: print error message for invalid mount options

Overlayfs should print an error message if an incorrect mount option
is caught like other filesystems.

After this patch, improper option input could be clearly known.

Reported-by: Fabian Sturm <fabian.sturm@aduu.de>
Signed-off-by: hujianyang <hujianyang@huawei.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>