Improve comments in nd6_dad_start()

[hiren]

Summary

Add a condition for when to not do DAD and rearrange for better readability.

[hrs]

• The state "tentative" is for an address, not an interface.
• nd6_dad_start() is not supposed to be called for an address with no "tentative" flag. If it happens, it is just a bug or something goes wrong. So it is a bit strange to have it as a comment of "cases when no DAD required".

[hiren]

In D1739#3, @hrs wrote:

• The state "tentative" is for an address, not an interface.
• nd6_dad_start() is not supposed to be called for an address with no "tentative" flag. If it happens, it is just a bug or something goes wrong. So it is a bit strange to have it as a comment of "cases when no DAD required".

I understand that part but I am just trying to match 3 checks we do with comments on the top to make the code more verbose.

Any better way to word it?

[hrs]

In D1739#4, @hiren wrote:

I understand that part but I am just trying to match 3 checks we do with comments on the top to make the code more verbose.

Any better way to word it?

So please add a sentence which explains the "tentative" case is not expected to happen and the check has been added for debugging purpose, while the other two cases are expected in normal situation.