percpu: misc preparations for nommu support

Make the following misc preparations for percpu nommu support.

* Remove refernces to vmalloc in common comments as nommu percpu won't
  use it.

* Rename chunk->vms to chunk->data and make it void *.  Its use is
  determined by chunk management implementation.

* Relocate utility functions and add __maybe_unused to functions which
  might not be used by different chunk management implementations.

This patch doesn't cause any functional change.  This is to allow
alternate chunk management implementation for percpu nommu support.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Howells <dhowells@redhat.com>
Cc: Graff Yang <graff.yang@gmail.com>
Cc: Sonic Zhang <sonic.adi@gmail.com>