fix devtmpfs race

After we's done complete(&req->done), there's nothing to prevent the
scope containing *req from being gone and *req overwritten by any
kind of junk.  So we must read req->next before that...
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

drivers/base/devtmpfs.c

@@ -406,9 +406,10 @@ static int devtmpfsd(void *p)
 			requests = NULL;
 			spin_unlock(&req_lock);
 			while (req) {
+				struct req *next = req->next;
 				req->err = handle(req->name, req->mode, req->dev);
 				complete(&req->done);
-				req = req->next;
+				req = next;
 			}
 			spin_lock(&req_lock);
 		}