Fixup last build warning

/include/linux/rbtree.h:125:27: warning: '*((void *)&waiter+12).rb_parent_color' is used uninitialized in this function [-Wuninitialized]
  rb->rb_parent_color = (rb->rb_parent_color & 3) | (unsigned long)p;
                           ^
/kernel/rtmutex.c:724:25: note: 'waiter' was declared here
  struct rt_mutex_waiter waiter;