percpu: reduce PCPU_MIN_UNIT_SIZE to 32k

In preparation of enabling percpu allocator for UP, reduce
PCPU_MIN_UNIT_SIZE to 32k.  On UP, the first chunk doesn't have to
include static percpu variables and chunk size can be smaller which is
important as UP percpu allocator will use contiguous kernel memory to
populate chunks.

PCPU_MIN_UNIT_SIZE also determines the maximum supported allocation
size but 32k should still be enough.

Signed-off-by: Tejun Heo <tj@kernel.org>