sched: fix fastcall mismatch in completion APIs

Jeff Dike noticed that wait_for_completion_interruptible()'s prototype
had a mismatched fastcall.

Fix this by removing the fastcall attributes from all the completion APIs.

Found-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>