MAINTAINERS: Add lib/percpu* as part of percpu entry

Without this patch get_maintainers.pl on a patch which modified
lib/percpu_refcount.c produces:

Jens Axboe <axboe@kernel.dk> (commit_signer:2/5=40%)
Ming Lei <ming.lei@redhat.com> (commit_signer:2/5=40%,authored:2/5=40%,added_lines:99/114=87%,removed_lines:34/43=79%)
"Paul E. McKenney" <paulmck@kernel.org> (commit_signer:1/5=20%,authored:1/5=20%,added_lines:9/114=8%,removed_lines:3/43=7%)
Tejun Heo <tj@kernel.org> (commit_signer:1/5=20%)
Andrew Morton <akpm@linux-foundation.org> (commit_signer:1/5=20%)
Nikolay Borisov <nborisov@suse.com> (authored:1/5=20%,removed_lines:3/43=7%)
Joe Perches <joe@perches.com> (authored:1/5=20%,removed_lines:3/43=7%)
linux-kernel@vger.kernel.org (open list)

Whereas with the patch applied it now (properly) prints:

Dennis Zhou <dennis@kernel.org> (maintainer:PER-CPU MEMORY ALLOCATOR)
Tejun Heo <tj@kernel.org> (maintainer:PER-CPU MEMORY ALLOCATOR)
Christoph Lameter <cl@linux.com> (maintainer:PER-CPU MEMORY ALLOCATOR)
linux-kernel@vger.kernel.org (open list)

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
[Dennis: updated list to linux-mm@kvack.org]
Signed-off-by: Dennis Zhou <dennis@kernel.org>