Fix a witness assertion that fires when a lock type's class changes.

When all instances of a lock type are destroyed (for example, after a
module unload), the corresponding witness entry remains associated with
that lock type. In this case, we shouldn't panic if a new instance of the
lock type is created and its lock class does not match that recorded in the
witness entry.

Reviewed by: jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D11788