ktrace/kern_exec: check p_tracecred instead of p_cred
ktrace/kern_exec: check p_tracecred instead of p_cred

.. when deciding whether to continue tracing across suid/sgid exec.
Otherwise if root ktrace-d an unprivileged process and the processed
exec-ed a suid program, then tracing didn't continue across exec.

Reviewed by:	bde, kib
MFC after:	22 days

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase