sched/fair: Fix per-CPU kthread and wakee stacking for asym CPU capacity

select_idle_sibling() has a special case for tasks woken up by a per-CPU
kthread. For this case, the chosen CPU is the previous one. This is an
issue for asymmetric CPU capacity systems where the wakee might not fit
that CPU anymore. Evaluate asym_fits_capacity() for prev_cpu before using
the exit path described above.

Fixes: b4c9c9f1 ("sched/fair: Prefer prev cpu in asymmetric wakeup path")
Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>