kernel: make sure thread priority is within valid range

The scheduler expects that all threads expect the initial idle threads
have priority in range [THREAD_MIN_SET_PRIORITY, THREAD_MAX_SET_PRIORITY].
If the requested pririty is out of range the value is clamped. Failing
with B_BAD_VALUE is probably an overkill since there isn't any real
change in the guarantees provided by the scheduler about the behavior
of such thread. Also, BeBook suggests that spawn_thread() can specify
priority 0.

src/system/kernel/thread.cpp

@@ -891,6 +891,10 @@ thread_create_thread(const ThreadCreationAttributes& attributes, bool kernel)
 		// available for deinitialization
 	thread->priority = attributes.priority == -1
 		? B_NORMAL_PRIORITY : attributes.priority;
+	thread->priority = std::max(thread->priority,
+			(int32)THREAD_MIN_SET_PRIORITY);
+	thread->priority = std::min(thread->priority,
+			(int32)THREAD_MAX_SET_PRIORITY);
 	thread->state = B_THREAD_SUSPENDED;
 
 	thread->sig_block_mask = attributes.signal_mask;