sched_ext: Split schedule_deferred() into locked and unlocked variants

sched_ext: Split schedule_deferred() into locked and unlocked variants

schedule_deferred() currently requires the rq lock to be held so that it can
use scheduler hooks for efficiency when available. However, there are cases
where deferred actions need to be scheduled from contexts that don't hold the
rq lock.

Split into schedule_deferred() which can be called from any context and just
queues irq_work, and schedule_deferred_locked() which requires the rq lock and
can optimize by using scheduler hooks when available. Update the existing call
site to use the _locked variant.

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Tejun Heo <tj@kernel.org>