Change knlist_destroy() to assert that knlist is empty instead of
accepting the wrong state and printing warning. Do not obliterate
kl_lock and kl_unlock pointers, they are often useful for post-mortem
analysis.

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
X-Differential revision: https://reviews.freebsd.org/D8898