sched_ext: Report a sub-scheduler's attach-time ecaps before lifting its enable bypass

A sub-scheduler learns which cids it holds through ops.sub_ecaps_updated().
Today, the grants its parent makes when it attaches are reported only after
its enable bypass is lifted, which is also when its tasks are handed to it.
So the sub-scheduler gets its first ops.enqueue() calls before it has been
notified of a single cid and has to hold the tasks somewhere until the
grants arrive.

Report the attach-time grants before lifting bypass instead. The
sub-scheduler then has its complete cid view when the first task arrives and
can place it right away. Nothing else reaches its ops in the meantime, as it
is still bypassing.

Signed-off-by: Tejun Heo <tj@kernel.org>