audit: correct reporting of *execve(2) success

r326145 corrected do_execve() to return EJUSTRETURN upon success so that
important registers are not clobbered. This had the side effect of tapping
out 'failures' for all *execve(2) audit records, which is less than useful
for auditing purposes.

Audit exec returns earlier, where we can know for sure that EJUSTRETURN
translates to success. Note that this unsets TDP_AUDITREC as we commit the
audit record, so the usual audit in the syscall return path will do nothing.

PR: 249179
Reported by: Eirik Oeverby <ltning-freebsd anduin net>
Reviewed by: csjp, kib
MFC after: 1 week
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D26922