sched/deadline: Make dl_cpuset_cpumask_can_shrink() capacity-aware

dl_cpuset_cpumask_can_shrink() is used to validate whether there is
still enough CPU capacity for DL tasks in the reduced cpuset.

Currently it still operates on `# remaining CPUs in the cpuset` (1).
Change this to use the already capacity-aware DL admission control
__dl_overflow() for the `cpumask can shrink` test.

  dl_b->bw = sched_rt_period << BW_SHIFT / sched_rt_period

  dl_b->bw * (1) >= currently allocated bandwidth in root_domain (rd)

  Replace (1) w/ `\Sum CPU capacity in rd >> SCHED_CAPACITY_SHIFT`

Adapt __dl_bw_capacity() to take a cpumask instead of a CPU number
argument so that `rd->span` and `cpumask of the reduced cpuset` can
be used here.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>