tools/mdflush: Fix CI build failure

[xingfeng2510]

CI failed due to the following error (in mdflush.py, bio->bi_disk not found). I guess CI runtime env (such as kernel version) had changed in recent days.

error: no member named 'bi_disk' in 'struct bio'
     struct gendisk *bi_disk = bio->bi_disk;
                              ~~~  ^
1 error generated.
Traceback (most recent call last):
   File "/bcc/tools/mdflush.py", line 54, in <module>

Before kernel 4.14, stay the same, just use bio->bi_bdev->bd_disk.
Since kernel 4.14, macro bio_dev begin to appear, which refers to bio->bi_disk.
Since kernel 5.12, macro bio_dev has changed to bio->bi_bdev->bd_disk (bio->bi_bdev changed back). So, just use '#ifdef bio_dev' could not make the right choice under higher kernel version.

This patch try to fix this error. Please take a look. @davemarchevsky @yonghong-song

[davemarchevsky]

@xingfeng2510 thank you for fixing this! I really appreciate your recent CI fixes

[xingfeng2510]

@davemarchevsky My pleasure, It's very kind of you.