From: Rusty Russell <rusty@rustcorp.com.au>

OK, this does the *minimum* required to support DEFINE_PER_CPU inside
modules.  If we decide to change kmalloc_percpu later, great, we can turf
this out.

Basically, overallocates the amount of per-cpu data at boot to at least
PERCPU_ENOUGH_ROOM if CONFIG_MODULES=y (arch-specific by default 32k: I have
only 7744 bytes of percpu data in my kernel here, so makes sense), and a
special allocator in module.c dishes it out.