percpu: use pcpu_free_slot instead of pcpu_nr_slots - 1

This prepares for adding a to_depopulate list and sidelined list after
the free slot in the set of lists in pcpu_slot.

Signed-off-by: Dennis Zhou <dennis@kernel.org>
Acked-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>