io_uring/rsrc: remove node assignment helpers

There are two helpers here, one assigns and increments the node ref
count, and the other is simply a wrapper around that for the buffer node
handling.

The buffer node assignment benefits from checking and setting
REQ_F_BUF_NODE together, otherwise stalls have been observed on setting
that flag later in the process. Hence re-do it so that it's set when
checked, and cleared in case of (unlikely) failure. With that, the
buffer node helper can go, and then drop the generic
io_req_assign_rsrc_node() helper as well as there's only a single user
of it left at that point.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/io_uring.c

@@ -1912,7 +1912,8 @@ inline struct file *io_file_get_fixed(struct io_kiocb *req, int fd,
 	io_ring_submit_lock(ctx, issue_flags);
 	node = io_rsrc_node_lookup(&ctx->file_table.data, fd);
 	if (node) {
-		io_req_assign_rsrc_node(&req->file_node, node);
+		node->refs++;
+		req->file_node = node;
 		req->flags |= io_slot_flags(node);
 		file = io_slot_file(node);
 	}