ovl: check that upperdir path is not on a read-only mount

So far we only checked that sb is not read-only.

Suggested-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>