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>