[kernel] Add lock-releasing version of thread::block()

Add a version of thread::block() that takes a lock and releases it after
marking the thread as unready, but before calling the scheduler.

Use this version of block() in the wait_queue.

src/kernel/objects/thread.cpp

@@ -52,6 +52,18 @@ thread::block()
     return m_wake_value;
 }
 
+uint64_t
+thread::block(util::scoped_lock &lock)
+{
+    kassert(current_cpu().thread == this,
+            "unlocking block() called on non-current thread");
+
+    clear_state(state::ready);
+    lock.release();
+    scheduler::get().schedule();
+    return m_wake_value;
+}
+
 j6_status_t
 thread::join()
 {

src/kernel/objects/thread.h

@@ -96,9 +96,14 @@ public:
     inline void set_priority(uint8_t p) { if (!constant()) m_tcb.priority = p; }
 
     /// Block this thread, waiting for a value
-    /// \returns  The value passed to wake()
+    /// \returns   The value passed to wake()
     uint64_t block();
 
+    /// Block this thread, waiting for a value
+    /// \arg held  A held lock to unlock when blocking
+    /// \returns   The value passed to wake()
+    uint64_t block(util::scoped_lock &held);
+
     /// Block the calling thread until this thread exits
     j6_status_t join();