MDEV-17640 UMASK_DIR configuration for mysql_install_db is not applied to mysql database

regression after 3db6de33

in RPMs continue creating the $datadir outside of mysql_install_db.
RPMs put the socket in there, so it cannot be chmod 0700.

support-files/rpm/server-postin.sh

@@ -53,6 +53,9 @@ if [ $1 = 1 ] ; then
   chown -R %{mysqld_user}:%{mysqld_group} $datadir
 
   if [ ! -e $datadir/mysql ]; then
+    # Create data directory
+    mkdir -p $datadir
+
     # Initiate databases
     %{_bindir}/mysql_install_db --rpm --user=%{mysqld_user}
   fi