mdflush: fix for Linux 4.14

Starting Linux 4.14, there's one less indirection layer to get the
disk name. Use the bio_dev macro to discriminate between versions.

tools/mdflush.py

@@ -36,8 +36,19 @@
  u32 pid = bpf_get_current_pid_tgid();
  data.pid = pid;
  bpf_get_current_comm(&data.comm, sizeof(data.comm));
+/*
+ * The following deals with a kernel version change (in mainline 4.14, although
+ * it may be backported to earlier kernels) with how the disk name is accessed.
+ * We handle both pre- and post-change versions here. Please avoid kernel
+ * version tests like this as much as possible: they inflate the code, test,
+ * and maintenance burden.
+ */
+#ifdef bio_dev
+ bpf_probe_read(&data.disk, sizeof(data.disk), bio->bi_disk->disk_name);
+#else
  bpf_probe_read(&data.disk, sizeof(data.disk),
- bio->bi_bdev->bd_disk->disk_name);
+ bio->bi_bdev->bd_disk->disk_name);
+#endif
  events.perf_submit(ctx, &data, sizeof(data));
  return 0;
 }