futex,rt_mutex: Introduce rt_mutex_init_waiter()

commit 50809358dd7199aa7ce232f6877dd09ec30ef374 upstream.

Since there's already two copies of this code, introduce a helper now
before adding a third one.

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/20170322104151.950039479@infradead.org


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[bwh: Backported to 4.9: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>