sched_ext: Fix stray scx_root usage in task_can_run_on_remote_rq()

task_can_run_on_remote_rq() takes @sch but it is using scx_root when
incrementing SCX_EV_DISPATCH_LOCAL_DSQ_OFFLINE, which is inconsistent and
gets in the way of implementing multiple scheduler support. Use @sch
instead. As currently scx_root is the only possible scheduler instance, this
doesn't cause any behavior changes.

Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>