mm/balloon_compaction: make balloon page compaction callbacks static

mm/balloon_compaction: make balloon page compaction callbacks static

Since commit b1123ea6d3b3 ("mm: balloon: use general non-lru movable page
feature"), these functions are called via balloon_aops callbacks.  They're
not called directly outside this file.  So make them static and clean up
the relevant code.

Link: https://lkml.kernel.org/r/20220125132221.2220-1-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Rafael Aquini <aquini@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>