While thinking about precedence of conflicting block device options from
different sources, I noticed that you can specify both an option and its
legacy alias at the same time (e.g. readonly=on,read-only=off). Rather
than specifying the order of precedence, we should simply forbid such
combinations.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Benoît Canet <benoit.canet@nodalink.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>