futex: Change locking rules

Currently futex-pi relies on hb->lock to serialize everything. But hb->lock
creates another set of problems, especially priority inversions on RT where
hb->lock becomes a rt_mutex itself.

The rt_mutex::wait_lock is the most obvious protection for keeping the
futex user space value and the kernel internal pi_state in sync.

Rework and document the locking so rt_mutex::wait_lock is held accross all
operations which modify the user space value and the pi state.

This allows to invoke rt_mutex_unlock() (including deboost) without holding
hb->lock as a next step.

Nothing yet relies on the new locking rules.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: juri.lelli@arm.com
Cc: bigeasy@linutronix.de
Cc: xlpang@redhat.com
Cc: rostedt@goodmis.org
Cc: mathieu.desnoyers@efficios.com
Cc: jdesfossez@efficios.com
Cc: dvhart@infradead.org
Cc: bristot@redhat.com
Link: http://lkml.kernel.org/r/20170322104151.751993333@infradead.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

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