io_uring: safer fallback_work free

Add extra wq flushing for fallback_work, that's not necessary but safer
if invariants of io_fallback_req_func() change.
Signed-off-by: NPavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/24179419d6748516299600bc914f50b9e0b02275.1633532552.git.asml.silence@gmail.comSigned-off-by: NJens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1262,7 +1262,6 @@ static __cold void io_fallback_req_func(struct work_struct *work)
 		mutex_unlock(&ctx->uring_lock);
 	}
 	percpu_ref_put(&ctx->refs);
-
 }
 
 static __cold struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
@@ -9212,6 +9211,7 @@ static __cold void io_ring_ctx_free(struct io_ring_ctx *ctx)
 	if (ctx->rsrc_backup_node)
 		io_rsrc_node_destroy(ctx->rsrc_backup_node);
 	flush_delayed_work(&ctx->rsrc_put_work);
+	flush_delayed_work(&ctx->fallback_work);
 
 	WARN_ON_ONCE(!list_empty(&ctx->rsrc_ref_list));
 	WARN_ON_ONCE(!llist_empty(&ctx->rsrc_put_llist));