DEBUG_VFS_LOCKS: use witness if available

The assert_vop_locked messages are ignored, and file/line information
is not too useful. Fixing this without changing both witness and VFS
asserts KPIs is not possible.

Reviewed by: markj (previous version)
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D39464