mariadb: Check and fix datadir owner issues during upgrade

If you are migrating to MariaDB package, you might have old datadir
stored somewhere using different setup with different users. If you
trust us enough to enable autoupgrade, you probably trust us enough to
chown your datadir as well. This can prevent some potential issues.

Signed-off-by: Michal Hrusecky <michal.hrusecky@turris.com>