Redox: Fix Condvar.wait(); do not lock mutex twice
Redox: Fix Condvar.wait(); do not lock mutex twice

The atomic_xchg() loop locks the mutex, so the call to mutex_lock is
incorrect, and blocks.

GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust