sched: add wait_for_completion_killable_timeout

Add missing _killable_timeout variant for wait_for_completion that will
return when a timeout expires or the task is killed.

CC: Ingo Molnar <mingo@elte.hu>
CC: Andreas Herrmann <andreas.herrmann3@amd.com>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Mike Galbraith <efault@gmx.de>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Sage Weil <sage@newdream.net>
diff --git a/include/linux/completion.h b/include/linux/completion.h
index 4a6b604..51e3145 100644
--- a/include/linux/completion.h
+++ b/include/linux/completion.h
@@ -83,6 +83,8 @@
 						   unsigned long timeout);
 extern unsigned long wait_for_completion_interruptible_timeout(
 			struct completion *x, unsigned long timeout);
+extern unsigned long wait_for_completion_killable_timeout(
+			struct completion *x, unsigned long timeout);
 extern bool try_wait_for_completion(struct completion *x);
 extern bool completion_done(struct completion *x);