Impact: allow unit_size to be arbitrary multiple of PAGE_SIZE

In dynamic percpu allocator, there is no reason the unit size should
be power of two.  Remove the restriction.

As non-power-of-two unit size means that empty chunks fall into the
same slot index as lightly occupied chunks which is bad for reclaming.
Reserve an extra slot for empty chunks.
Signed-off-by: Tejun Heo <tj@kernel.org>