ovl: Prevent rw remount when it should be ro mount

Overlayfs should be mounted read-only when upper-fs is read-only or nonexistent.
But now it can be remounted read-write and this can cause kernel panic.
So we should prevent read-write remount when the above situation happens.

Signed-off-by: Seunghun Lee <waydi1@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>