percpu: add __percpu for sparse

This is to make the annotation of percpu variables during the next merge
window less painfull.

Extracted from a patch by Rusty Russell.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>