MDEV-18059 `support-files/mysql.server.sh stop` must run as root

don't run `su - mysql` is $USER is already mysql

support-files/mysql.server.sh

@@ -187,7 +187,11 @@ fi
 user='@MYSQLD_USER@'
 
 su_kill() {
-  su - $user -s /bin/sh -c "kill $*" >/dev/null 2>&1
+  if test "$USER" = "$user"; then
+    kill $* >/dev/null 2>&1
+  else
+    su - $user -s /bin/sh -c "kill $*" >/dev/null 2>&1
+  fi
 }
 
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