Add mdas_empty_or_ignored() helper function.

Add mdas_empty_or_ignored() helper function.

Add a helper function to consolidate checking for an empty mdas list
or ignored mdas.  Ignored mdas should behave almost identically to
an empty mda list - the metadata areas should not be read or written
to.  This function will make it easier to implement metadata balancing
and easier to track pvs with an empty mda list or ignored mdas.

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>