mm/filemap: make folio_put_wait_locked static

It's only used in mm/filemap.c, since commit <ffa65753>
("mm/migrate.c: rework migration_entry_wait() to not take a pageref").

Make it static.

Link: https://lkml.kernel.org/r/20220914021738.3228011-1-sunke@kylinos.cn


Signed-off-by: Ke Sun <sunke@kylinos.cn>
Reported-by: k2ci <kernel-bot@kylinos.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>