selftests/sched_ext: Validate select_cpu_and mask constraints

The syscall test only prints the CPU selection result after checking
whether BPF_PROG_TEST_RUN succeeds.

Require -EBUSY for empty and affinity-disjoint custom masks, and allow
a legal singleton candidate to be selected or busy. Reject unexpected
errors and selections outside the custom mask or configured affinity.
Check cpus_mask because migration disabling can temporarily narrow
cpus_ptr under CONFIG_PREEMPT_RCU.

Grow dynamically allocated affinity masks until sched_getaffinity()
succeeds, retaining that allocation size for pinning and restoration.
Restore affinity on all exits after pinning, and skip only the disjoint
case when fewer than two CPUs are allowed.

Assisted-by: LLM
Signed-off-by: Tianyi Chen <hi@tychen.cc>
Signed-off-by: Tejun Heo <tj@kernel.org>

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