mm: constify page_count and page_ref_count

Now that compound_head() accepts a const struct page pointer, these two
functions can be marked as not modifying the page pointer they are passed.

Link: https://lkml.kernel.org/r/20210416231531.2521383-7-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/page_ref.h

@@ -62,12 +62,12 @@ static inline void __page_ref_unfreeze(struct page *page, int v)
 
 #endif
 
-static inline int page_ref_count(struct page *page)
+static inline int page_ref_count(const struct page *page)
 {
 	return atomic_read(&page->_refcount);
 }
 
-static inline int page_count(struct page *page)
+static inline int page_count(const struct page *page)
 {
 	return atomic_read(&compound_head(page)->_refcount);
 }