sched_ext: scx_qmap: Place only on cids whose caps are in effect

qmap decides placements from self_cids, which redistribute() derives from
the caps view at ops.sub_caps_updated() time. That view runs ahead of the
cpus: a granted cid can be in self_cids before its cpu has reported the caps
in effect through ops.sub_ecaps_updated(). ops.update_idle() only comes once
BASE is in effect, so the idle-gated placements reach such a cid only
through an idle bit left over from an earlier hold. The highpri scan has no
gate at all:

  parent                    cpu Y, qmap               cpu X
  grants ENQ on X to qmap
                            sub_caps_updated() adds X
                            to self_cids
                            highpri scan moves a task
                            to X with PREEMPT
                                                      caps not in effect,
                                                      move denied, task
                                                      bounced with REENQ_CAP
                                                      reject drain, enqueue
                            the scan moves it to X
                            again
                                                      denied again
                                                      dispatch syncs ecaps,
                                                      sub_ecaps_updated(X)

Every highpri move to X in that window is denied and bounced. The two
callbacks are meant to split the roles: ops.sub_caps_updated() tracks what
the node holds and drives what it delegates to its children, while
ops.sub_ecaps_updated() says whether a task can run on a cpu now. qmap used
the first for both. Track the caps in effect from ops.sub_ecaps_updated() as
avail_cids and place only on self_cids & avail_cids, so that self_cids stays
the delegation split and avail_cids gates the placement.

The stranded tests keep self_cids, as they ask whether the split gives the
task anywhere at all. A highpri task whose self_cids lack caps in effect
waits for them instead of being moved and bounced.

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

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