sched_ext: Add an event, SCX_EV_BYPASS_DISPATCH

Add a core event, SCX_EV_BYPASS_DISPATCH, which represents how many
tasks have been dispatched in the bypass mode.

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

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