mm/page_ext.c: constify lookup_page_ext() argument

lookup_page_ext() finds 'struct page_ext' for a given page.  It requires
only read access to the given struct page.

Current implemnentation takes 'struct page *' as an argument.  It makes
compiler complain when 'const struct page *' passed.

Change the argument to 'const struct page *'.

Link: http://lkml.kernel.org/r/20180531135457.20167-3-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Vinayak Menon <vinmenon@codeaurora.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/page_ext.h

@@ -50,7 +50,7 @@ static inline void page_ext_init(void)
 }
 #endif
 
-struct page_ext *lookup_page_ext(struct page *page);
+struct page_ext *lookup_page_ext(const struct page *page);
 
 #else /* !CONFIG_PAGE_EXTENSION */
 struct page_ext;
@@ -59,7 +59,7 @@ static inline void pgdat_page_ext_init(struct pglist_data *pgdat)
 {
 }
 
-static inline struct page_ext *lookup_page_ext(struct page *page)
+static inline struct page_ext *lookup_page_ext(const struct page *page)
 {
 	return NULL;
 }

mm/page_ext.c

@@ -120,7 +120,7 @@ void __meminit pgdat_page_ext_init(struct pglist_data *pgdat)
 	pgdat->node_page_ext = NULL;
 }
 
-struct page_ext *lookup_page_ext(struct page *page)
+struct page_ext *lookup_page_ext(const struct page *page)
 {
 	unsigned long pfn = page_to_pfn(page);
 	unsigned long index;
@@ -195,7 +195,7 @@ void __init page_ext_init_flatmem(void)
 
 #else /* CONFIG_FLAT_NODE_MEM_MAP */
 
-struct page_ext *lookup_page_ext(struct page *page)
+struct page_ext *lookup_page_ext(const struct page *page)
 {
 	unsigned long pfn = page_to_pfn(page);
 	struct mem_section *section = __pfn_to_section(pfn);