sched_ext: Add an event, SCX_EV_DISPATCH_KEEP_LAST

Add a core event, SCX_EV_DISPATCH_KEEP_LAST, which represents how many
times a task is continued to run without ops.enqueue() when
SCX_OPS_ENQ_LAST is not set.

__scx_add_event() is used since the caller holds an rq lock,
so the preemption has already been disabled.

Signed-off-by: Changwoo Min <changwoo@igalia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>