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.

Signed-off-by: Miaohe Lin <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Muchun Song <[email protected]>