sched: core: fixed a lock bug in rq_destroy_kill_q

rq_destroy_kill_q is responsible for killing all threads in the kill
queue of a run queue (usually not more then one). However, it is still
important that this happens securely, while the run queue is locked.
This process wasn't happening securely up untill now. The run queue lock
is release by rq_schedule (the caller of this function), just before it
calls rq_destroy_kill_q.

Fixed by adding lock calls to rq_destroy_kill_q.

Signed-off-by: Michel Megens <dev@michelmegens.net>