fix: optional fields for /proc/self/mountinfo

[flisky]

according to https://man7.org/linux/man-pages/man5/proc.5.html,
/proc/[pid]/mountinfo could have zero or more
fields and separated by a single hyphen.

[seanmonstar]

Thanks for the PR! Do you have an example that this fixes, to help me understand the reason for the code changes?

[flisky]

Thanks for investigating this!

       /proc/[pid]/mountinfo (since Linux 2.6.26)
              ...
              The file contains lines of the form:

              36 35 98:0 /mnt1 /mnt2 rw,noatime master:1 - ext3 /dev/root rw,errors=continue
              (1)(2)(3)   (4)   (5)      (6)      (7)   (8) (9)   (10)         (11)

On our k8s instance, the mountinfo has none optional fields (part (7) in above form).
(Actually, it is sentenced as a private mount)

However, it's asked for exactly one optional field in the current implementation (shared:7 in fixtures).

[seanmonstar]

Oh ok, cool. Speaking of, can this make use of the existing fixtures (or add a new one if needed) to make some tests to catch this fix?

[flisky]

Fixture is a bit cumbersome in this case, and I prefer some simple unittest :)

[seanmonstar]

Awesome, thank you for the tests! And now I've just realized that this repo is still setup for Travis-CI, and that's no longer free. I need to get the tests ported over to GitHub Actions...

[flisky]

#113 was merged, closing.