A comment in debian/mariadb-server-10.1.postinst says: "can safely run on
upgrades with existing databases". While this is true there're a few reasons not
to do that:
- it increases installation time (it has to run rather heavy mysqld multiple times)
- as well as it increases mysqld downtime
- it may fail if database has some plugin specific configs (see MDEV-8437)
- there should be no need to run this script on upgrade: they should be handled
  by mysql_upgrade to
- RPM postin doesn't call it if database directory exists

Also postinst is not supposed to create database directories: let
mysql_install_db do that intead.