Reapply fix for bug#16372 (Server crashes when test 'conc_sys' is running)

after merge.

Concurrent read and update of privilege structures (like simultaneous
run of SHOW GRANTS and ADD USER) could result in server crash.

Ensure that proper locking of ACL structures is done.

No test case is provided because this bug can't be reproduced
deterministically.

sql/sql_acl.h

@@ -196,7 +196,6 @@ bool mysql_table_grant(THD *thd, TABLE_LIST *table, List <LEX_USER> &user_list,
 bool mysql_routine_grant(THD *thd, TABLE_LIST *table, bool is_proc,
 			 List <LEX_USER> &user_list, ulong rights,
 			 bool revoke, bool no_error);
-ACL_USER *check_acl_user(LEX_USER *user_name, uint *acl_acl_userdx);
 my_bool grant_init();
 void grant_free(void);
 my_bool grant_reload(THD *thd);

sql/sql_parse.cc

@@ -3835,7 +3835,6 @@ mysql_execute_command(THD *thd)
     if (thd->security_ctx->user)              // If not replication
     {
       LEX_USER *user;
-      uint counter;
 
       List_iterator <LEX_USER> user_list(lex->users_list);
       while ((user= user_list++))
@@ -3853,7 +3852,8 @@ mysql_execute_command(THD *thd)
                           user->host.str, thd->security_ctx->host_or_ip))
         {
           // TODO: use check_change_password()
-          if (check_acl_user(user, &counter) && user->password.str &&
+          if (is_acl_user(user->host.str, user->user.str) &&
+              user->password.str &&
               check_access(thd, UPDATE_ACL,"mysql",0,1,1,0))
           {
             my_message(ER_PASSWORD_NOT_ALLOWED,