khelp: suppress useless warning message on shutdown

If a module (e.g. the ertt hhook for TCP) can't clean up at
shutdown, there is nothing to be done about it. In the ertt case,
cleanup is just shutting down a UMA zone, which doesn't need to be
done. Suppress EBUSY warnings on shutdown.

PR: 271677
Reviewed by: tuexen, imp
Differential Revision: https://reviews.freebsd.org/D42650

(cherry picked from commit 415c1c748d5492e41328fedf96b6bf3c9be94595)