Robust mutexes: Fix lost wake-up.

Assume that Thread 1 waits to acquire a robust mutex using futexes to
block (and thus sets the FUTEX_WAITERS flag), and is unblocked when this
mutex is released.  If Thread 2 concurrently acquires the lock and is
killed, Thread 1 can recover from the died owner but fail to restore the
FUTEX_WAITERS flag.  This can lead to a Thread 3 that also blocked using
futexes at the same time as Thread 1 to not get woken up because
FUTEX_WAITERS is not set anymore.

The fix for this is to ensure that we continue to preserve the
FUTEX_WAITERS flag whenever we may have set it or shared it with another
thread.  This is the same requirement as in the algorithm for normal
mutexes, only that the robust mutexes need additional handling for died
owners and thus preserving the FUTEX_WAITERS flag cannot be done just in
the futex slowpath code.

	[BZ #20973]
	* nptl/pthread_mutex_lock.c (__pthread_mutex_lock_full): Fix lost
	wake-up in robust mutexes.
	* nptl/pthread_mutex_timedlock.c (pthread_mutex_timedlock): Likewise.

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