Fix "Bad tailq" panic when auditing auditon(A_SETCLASS, ...)

[asomers]

Summary

Fix "Bad tailq" panic when auditing auditon(A_SETCLASS, ...)

Due to an oversight in r195280, auditon(A_SETCLASS, ...) would cause a tailq
element to get added to the tailq twice, resulting in a circular tailq. This
panics when INVARIANTS are on.

Test Plan

Open /dev/auditpipe, set the preselect mode to LOCAL, set the preselect flags
and naflags to include class ad, then start auditd. Some ATF tests in progress
do this (Phabricator D15286).