NFS: null dereference in dev_remove()

In commit 5ffaf855 "NFS: replace global bl_wq with per-net one" we
made "msg" a pointer instead of a struct stored in stack memory.  But we
forgot to change the memset() here so we're still clearing stack memory
instead clearing the struct like we intended.  It will lead to a kernel
crash.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/blocklayout/blocklayoutdm.c

@@ -54,7 +54,7 @@ static void dev_remove(struct net *net, dev_t dev)
 	dprintk("Entering %s\n", __func__);
 
 	bl_pipe_msg.bl_wq = &nn->bl_wq;
-	memset(&msg, 0, sizeof(*msg));
+	memset(msg, 0, sizeof(*msg));
 	msg->data = kzalloc(1 + sizeof(bl_umount_request), GFP_NOFS);
 	if (!msg->data)
 		goto out;