MDEV-9165: Run chown much faster on the datadir during install/update

debian/mariadb-server-10.2.postinst

@@ -119,7 +119,7 @@ EOF
     # circumstances as it contains scripts that are executed by root.
     set +e
     chown -R 0:0 $mysql_statedir
-    chown -R mysql $mysql_datadir
+    find $mysql_datadir ! -uid $(id -u mysql) -print0 | xargs -0 -r chown mysql
     chown -R mysql:adm $mysql_logdir
     chmod 2750 $mysql_logdir
     set -e

debian/mariadb-server-10.2.preinst

@@ -199,7 +199,7 @@ fi
 # The "set +e" is necessary as e.g. a ".journal" of a ext3 partition is
 # not chgrp'able (#318435).
 set +e
-chown mysql:mysql $mysql_datadir
+find $mysql_datadir ! -uid $(id -u mysql) -print0 | xargs -0 -r chown mysql
 find $mysql_datadir -follow -not -group mysql -print0 2>/dev/null \
   | xargs -0 --no-run-if-empty chgrp mysql
 set -e