tsan: fix large stack frame in deadlock detector

In member function 'virtual void __sanitizer::DD::MutexBeforeLock(__sanitizer::DDCallback*, __sanitizer::DDMutex*, bool)':
error: the frame size of 544 bytes is larger than 512 bytes [-Werror=frame-larger-than=]

The code is now [arguably] better as well.

llvm-svn: 204227