audit: correct reporting of *execve(2) success

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