rump: check if the mutex is surely owned by the caller in mutex_exit

Unlocking a not-owned mutex wasn't detected well (it could detect if the mutex
is not held by anyone but that's not enough). Let's check it (the check is the
same as normal kernel's mutex).

If LOCKDEBUG is enabled, give the check over LOCKDEBUG because it can provide
better debugging information.

(ozaki-r)