mm: make __absent_pages_in_range() as static

It's only called in mm/mm_init.c now.

Link: https://lkml.kernel.org/r/20240326061134.1055295-4-bhe@redhat.com


Signed-off-by: Baoquan He <bhe@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: "Mike Rapoport (IBM)" <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>