report errors when attempting to reset a locked mount flag

Implement the same change present in runc since version 1.12 (https://github.com/opencontainers/runc/pull/3967).

Currently, when users attempts to clear locked flags (such as using rw to clear the MS_RDONLY flag), crun silently ignores the request, resulting in mounts remaining read-only.

So make sure that each request is either honored or an error message is returned.