In order to avoid a huge class of deadlocks (in particular in interactions
with the interlock), owner of the lock should be only curthread or at
least, for its limited usage, NULL which identifies LK_KERNPROC.

The thread "extra argument" for the lockmgr interface is going to be
removed in the near future, but for the moment, just let kernel run for
some days with this check on in order to find potential deadlocking
places around the kernel and fix them.