pagevec.h: add `const` to pointer parameters of getter functions

pagevec.h: add `const` to pointer parameters of getter functions

For improved const-correctness.

Link: https://lkml.kernel.org/r/20250828130311.772993-1-max.kellermann@ionos.com
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: SeongJae Park <sj@kernel.org>
Reviewed-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>