Replace somewhat awkward audit trail rotation scheme, which involved the global audit mutex and condition variables, with an sx lock which protects the trail vnode and credential while in use, and is acquired by the system call code when rotating the trail.  Previously, a "message" would be sent to the kernel audit worker, which did the rotation, but the new code is simpler and (hopefully) less error-prone.
Replace somewhat awkward audit trail rotation scheme, which involved the
global audit mutex and condition variables, with an sx lock which protects
the trail vnode and credential while in use, and is acquired by the system
call code when rotating the trail.  Previously, a "message" would be sent
to the kernel audit worker, which did the rotation, but the new code is
simpler and (hopefully) less error-prone.

Obtained from:	TrustedBSD Project
MFC after:	1 month

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