mtx: fix up _mtx_obtain_lock_fetch usage in thread lock

Since _mtx_obtain_lock_fetch no longer sets the argument to MTX_UNOWNED,
callers have to do it on their own.

sys/kern/kern_mutex.c

@@ -782,6 +782,7 @@ thread_lock_flags_(struct thread *td, int opts, const char *file, int line)
 #ifdef KDTRACE_HOOKS
 	spin_time -= lockstat_nsecs(&td->td_lock->lock_object);
 #endif
+	v = MTX_UNOWNED;
 	for (;;) {
 retry:
 		spinlock_enter();