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.

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D15381