MDEV-35663 Sporadic connection failures during FLUSH PRIVILEGES

during FLUSH PRIVILEGES, allow_all_hosts temporarily goes out of sync
with acl_check_hosts and acl_wild_hosts.

As it's tested in acl_check_host() without a mutex, let's re-test it
under a mutex to make sure the value is correct.

Note that it's just an optimization and it's ok to see outdated
allow_all_hosts value here.

Author: vuvova

sql/sql_acl.cc

@@ -3948,7 +3948,8 @@ bool acl_check_host(const char *host, const char *ip)
     return 0;
   mysql_mutex_lock(&acl_cache->lock);
 
-  if ((host && my_hash_search(&acl_check_hosts,(uchar*) host,strlen(host))) ||
+  if (allow_all_hosts ||
+      (host && my_hash_search(&acl_check_hosts,(uchar*) host,strlen(host))) ||
       (ip && my_hash_search(&acl_check_hosts,(uchar*) ip, strlen(ip))))
   {
     mysql_mutex_unlock(&acl_cache->lock);