Improve lock and unlock symmetry

• Move destruction of per-ring locks to netmap_dtor_locked to mirror the

initialization that happens in NIOCREGIF. Otherwise unloading a netmap-
capable interface that was never put into netmap mode would try to
mtx_destroy an uninitialized mutex, and panic.

• Destroy core_lock in netmap_detach, mirroring init in netmap_attach.

• Also comment out the knlist_destroy for now as there is currently no

knlist_init.

Sponsored by: ADARA Networks
Reviewed by: luigi@