kernel: Panic on attempts to block a pinned thread.

Preventing "normal" context switches caused by a time interrupt
is the primary reason for pinning threads. "thread_block" and friends,
however, cause an explicit context switch and will not return until
another thread unblocks us.

Calling these while a thread is pinned is thus undefined behavior,
and so we should just panic in the case anyone attempts to do so.

src/system/kernel/thread.cpp

@@ -2847,6 +2847,11 @@ thread_block_timeout(timer* timer)
 static inline status_t
 thread_block_locked(Thread* thread)
 {
+	if (thread->pinned_to_cpu > 0) {
+		panic("attempting to block thread %" B_PRId32 ", which is pinned!",
+			thread->id);
+	}
+
 	if (thread->wait.status == 1) {
 		// check for signals, if interruptible
 		if (thread_is_interrupted(thread, thread->wait.flags)) {