Fix RWSpinlock::promote_to_writer().
Fix RWSpinlock::promote_to_writer().

If multiple threads enter it, it will spin. Just fallback to unlock(),
lock() if we fail the first CAS.

Themaister/Granite,Themaister/Granite,Themaister/Granite,Themaister/Granite,Themaister/Granite,Themaister/Granite