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

Several tweaks to the kmalloc_percpu()/kfree_percpu() interface, to
allow future implementations to be more flexible, and make easier to
use now we can see how it's actually being used.

1) No flags argument: GFP_ATOMIC doesn't make much sense,

2) Explicit alignment argument, so we don't have to give SMP_CACHE_BYTES
   alignment always,

3) Zeros memory, since most callers want that and it's not entirely
   trivial,

4) Convenient type-safe wrapper which takes a typename, and

5) Rename to alloc_percpu/__alloc_percpu, since usage no longer matches
   kmalloc.