kernel - Fix lockmgr non-zero exclusive count panic

kernel - Fix lockmgr non-zero exclusive count panic

* The vm_map lock uses shared & exclusive locks and tries to upgrade
  shared to exclusive.  There is a race where a shared upgrade can
  steal an exclusive lock from an exclusive request which has already
  acquired the LK_WANT_EXCL flag.

* Deal with the case by having the exclusive lock also acquire
  LK_HAVE_EXCL to catch any shared upgrades which beat out the
  request.

Reported-by: YONETANI Tomokazu <qhwt.dfly@les.ath.cx>
Researched-by: YONETANI Tomokazu <qhwt.dfly@les.ath.cx>