percpu: fix too low alignment restriction on UP

UP __alloc_percpu() triggered WARN_ON_ONCE() if the requested
alignment is larger than that of unsigned long long, which is too
small for all the cacheline aligned allocations.  Bump it up to
SMP_CACHE_BYTES which kmalloc allocations generally guarantee.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Ingo Molnar <mingo@elte.hu>