mm: constify compound_order() and page_size()

Patch series "Small cleanups".

These small cleanups can be applied now to reduce conflicts during the
next merge window.  They're all from various efforts to split struct page
from other memdescs.  Thanks to Vlastimil for the suggestion.


This patch (of 3):

These functions do not modify their arguments.  Telling the compiler this
may improve code generation, and allows us to pass const arguments from
other functions.

Link: https://lkml.kernel.org/r/20250910142923.2465470-1-willy@infradead.org
Link: https://lkml.kernel.org/r/20250910142923.2465470-2-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>