kernel: Add an AssertLocked() to Team.

The mutex member is private, so this is necessary for other methods
to assert the team mutex is locked.

headers/private/kernel/thread_types.h

@@ -531,6 +531,8 @@ public:
 									{ return mutex_trylock(&fLock) == B_OK; }
 			void				Unlock()
 									{ mutex_unlock(&fLock); }
+			void				AssertLocked()
+									{ ASSERT_LOCKED_MUTEX(&fLock); }
 
 			void				UnlockAndReleaseReference()
 									{ Unlock(); ReleaseReference(); }