baseldap: de-duplicate passed attributes when checking for limits
    
    LDAP attribute options aren't enforced in the schema, thus we strip them
    when checking attribute conformance with the schema. This, however, can
    leave us with a situation when multiple base LDAP attribute names are
    present in the list of attribute names to check.
    
    Use set of attribute names to deduplicate the list.
    
    Fixes: https://pagure.io/freeipa/issue/8328
    
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>