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

regression after 3db6de3 in RPMs continue creating the $datadir outside of mysql_install_db. RPMs put the socket in there, so it cannot be chmod 0700.

Author: vuvova

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