rtmutex: Drop rt_mutex::wait_lock before scheduling

In the Linux kernel, the following vulnerability has been resolved:

rtmutex: Drop rt_mutex::wait_lock before scheduling

rt_mutex_handle_deadlock() is called with rt_mutex::wait_lock held. In the

good case it returns with the lock held and in the deadlock case it emits a

warning and goes into an endless scheduling loop with the lock held, which

triggers the 'scheduling in atomic' warning.

Unlock rt_mutex::wait_lock in the dead lock case before issuing the warning

and dropping into the schedule for ever loop.

[ tglx: Moved unlock before the WARN(), removed the pointless comment,

massaged changelog, added Fixes tag ]