io_uring: don't free request to slab

It's not necessary to free the request back to slab when we fail to
get sqe, just move it to state->free_list.

Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/20210825175856.194299-1-haoxu@linux.alibaba.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -6889,7 +6889,7 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr)
 		}
 		sqe = io_get_sqe(ctx);
 		if (unlikely(!sqe)) {
-			kmem_cache_free(req_cachep, req);
+			list_add(&req->inflight_entry, &ctx->submit_state.free_list);
 			break;
 		}
 		/* will complete beyond this point, count as submitted */