[PATCH] Trivial fix to aio.c:__aio_get_req()

This is a simple fix to aio.c:__aio_get_req() where it appears that
a freed aio request could be incorrectly returned in the error path,

fs/aio.c

@@ -400,8 +400,10 @@ static struct kiocb *__aio_get_req(struct kioctx *ctx)
 		req->ki_user_obj = NULL;
 		req->ki_ctx = ctx;
 		req->ki_users = 1;
-	} else
+	} else {
 		kmem_cache_free(kiocb_cachep, req);
+		req = NULL;
+	}
 	kunmap_atomic(ring, KM_USER0);
 	spin_unlock_irq(&ctx->ctx_lock);