rtld: clear any holes in the struct utrace_rtld passed to kernel logger

This should avoid an (almost) false positive from Valgrind, by filling
the padding on LP64.

PR: 284563
Reported by: Paul Floyd <pjfloyd@wanadoo.fr>
Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D48854