module: encapsulate percpu handling better and record percpu_size

Better encapsulate module static percpu area handling so that code
outsidef of CONFIG_SMP ifdef doesn't deal with mod->percpu directly
and add mod->percpu_size and record percpu_size in it.  Both percpu
fields are compiled out on UP.  While at it, mark mod->percpu w/
__percpu.

This is to prepare for is_module_percpu_address().

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>