Don't acquire evclass_lock with a spinlock held

When the "pc" audit class is enabled and auditd is running, witness will
panic during thread exit because au_event_class tries to lock an rwlock
while holding a spinlock acquired upstack by thread_exit.

To fix this, move AUDIT_SYSCALL_EXIT futher upstack, before the spinlock is
acquired. Of thread_exit's 16 callers, it's only necessary to call
AUDIT_SYSCALL_EXIT from two, exit1 (for exiting processes) and kern_thr_exit
(for exiting threads). The other callers are all kernel threads, which
needen't call AUDIT_SYSCALL_EXIT because since they can't make syscalls
there will be nothing to audit. And exit1 already does call
AUDIT_SYSCALL_EXIT, making the second call in thread_exit redundant for that
case.

PR: 228444
Reported by: aniketp
Reviewed by: aniketp, kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D16210