io_uring: don't miscount pinned memory

io_sqe_buffer_unregister() uses cxt->sqo_mm for memory accounting, but
io_ring_ctx_free() drops ->sqo_mm before leaving pinned_vm
over-accounted. Postpone mm cleanup for when it's not needed anymore.

Fixes: 3097582 ("io_uring: report pinned memory usage")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -7670,12 +7670,12 @@ static void io_destroy_buffers(struct io_ring_ctx *ctx)
 static void io_ring_ctx_free(struct io_ring_ctx *ctx)
 {
 	io_finish_async(ctx);
+	io_sqe_buffer_unregister(ctx);
 	if (ctx->sqo_mm) {
 		mmdrop(ctx->sqo_mm);
 		ctx->sqo_mm = NULL;
 	}
 
-	io_sqe_buffer_unregister(ctx);
 	io_sqe_files_unregister(ctx);
 	io_eventfd_unregister(ctx);
 	io_destroy_buffers(ctx);