Change knlist_destroy() to assert that knlist is empty instead of

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