percpu: fix spurious alignment WARN in legacy SMP percpu allocator

Impact: remove spurious WARN on legacy SMP percpu allocator

Commit f2a8205c incorrectly added too
tight WARN_ON_ONCE() on alignments for UP and legacy SMP percpu
allocator.  Commit e3176036 fixed it
for UP but legacy SMP allocator was forgotten.  Fix it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Sachin P. Sant <sachinp@in.ibm.com>

mm/allocpercpu.c

@@ -120,7 +120,7 @@ void *__alloc_percpu(size_t size, size_t align)
 	 * on it.  Larger alignment should only be used for module
 	 * percpu sections on SMP for which this path isn't used.
 	 */
-	WARN_ON_ONCE(align > __alignof__(unsigned long long));
+	WARN_ON_ONCE(align > SMP_CACHE_BYTES);
 
 	if (unlikely(!pdata))
 		return NULL;