carp: deal with negative net.inet.carp.demotion

[kp]

Summary

Given nodes 1 and 2, where node 1 has an advskew of 0 and node 2 has an
advskew of 100, making them master and backup respectively.

If net.inet.carp.demotion is set to a negative value on node 1, node 2
might become master while node 1 still retains it master status. Wether
or not node 2 becomes master seems to depend on the nodes advskew and
what the demotion sysctl was set to on node 1.

The reason for node 2 becoming master seems to be that the calculated
advskew taking demotion into account is truncated to a single unsigned
byte when copied into the carp header for sending, and node 1 stays
master since it takes uses the whole non-truncated calculated advskew
when deciding wether to stay master.

PR: 259528
Submitted by: [e-mail]
MFC after: 3 weeks
Sponsored by: Modirum MDPay

[donner]

Because both types are "int", the calculation does work.
Good catch!

[kp]

In D32759#739579, @donner wrote:

Good catch!

All bla^W credit to Marius.