md/raid5: fix new memory-reference bug in alloc_thread_groups.

In alloc_thread_groups, worker_groups is a pointer to an array,
not an array of pointers.
So
   worker_groups[i]
is wrong.  It should be
   &(*worker_groups)[i]

Found-by: coverity
Fixes: 60aaf9338545
Reported-by: Ben Hutchings <bhutchings@solarflare.com>
Cc: majianpeng <majianpeng@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -5471,7 +5471,7 @@ static int alloc_thread_groups(struct r5conf *conf, int cnt,
 	for (i = 0; i < *group_cnt; i++) {
 		struct r5worker_group *group;
 
-		group = worker_groups[i];
+		group = &(*worker_groups)[i];
 		INIT_LIST_HEAD(&group->handle_list);
 		group->conf = conf;
 		group->workers = workers + i * cnt;