io_uring/uring_cmd: fix buffer index retrieval

Add back buffer index retrieval for IORING_URING_CMD_FIXED.

Reported-by: Guangwu Zhang <guazhang@redhat.com>
Cc: Jeff Moyer <jmoyer@redhat.com>
Fixes: b54a140 ("io_uring/rsrc: add io_rsrc_node_lookup() helper")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Kanchan Joshi <joshi.k@samsung.com>
Reviewed-by: Anuj Gupta <anuj20.g@samsung.com>
Tested-by: Guangwu Zhang <guazhang@redhat.com>
Link: https://lore.kernel.org/r/20241111101318.1387557-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Ming Lei

io_uring/uring_cmd.c

@@ -210,8 +210,9 @@ int io_uring_cmd_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 	if (ioucmd->flags & IORING_URING_CMD_FIXED) {
 		struct io_ring_ctx *ctx = req->ctx;
 		struct io_rsrc_node *node;
+		u16 index = READ_ONCE(sqe->buf_index);
 
-		node = io_rsrc_node_lookup(&ctx->buf_table, req->buf_index);
+		node = io_rsrc_node_lookup(&ctx->buf_table, index);
 		if (unlikely(!node))
 			return -EFAULT;
 		/*