Kernel: Add a blunt big process lock.

We can't have multiple threads in the same process running in the kernel
at the same time, so let's have a per-process lock that threads have to
acquire on syscall entry/exit (and yield while blocked.)

Kernel/Lock.h

@@ -26,6 +26,7 @@ public:
 
     void lock();
     void unlock();
+    bool unlock_if_locked();
 
     const char* name() const { return m_name; }
 
@@ -90,6 +91,32 @@ inline void Lock::unlock()
     }
 }
 
+inline bool Lock::unlock_if_locked()
+{
+    for (;;) {
+        if (CAS(&m_lock, 1, 0) == 0) {
+            if (m_level == 0) {
+                memory_barrier();
+                m_lock = 0;
+                return false;
+            }
+            ASSERT(m_holder == current);
+            ASSERT(m_level);
+            --m_level;
+            if (m_level) {
+                memory_barrier();
+                m_lock = 0;
+                return false;
+            }
+            m_holder = nullptr;
+            memory_barrier();
+            m_lock = 0;
+            return true;
+        }
+        Scheduler::donate_to(m_holder, m_name);
+    }
+}
+
 #define LOCKER(lock) Locker locker(lock)
 
 template<typename T>