mm: introduce page_size()

Patch series "Make working with compound pages easier", v2.

These three patches add three helpers and convert the appropriate
places to use them.

This patch (of 3):

It's unnecessarily hard to find out the size of a potentially huge page.
Replace 'PAGE_SIZE << compound_order(page)' with page_size(page).

Link: http://lkml.kernel.org/r/20190721104612.19120-2-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/nommu.c b/mm/nommu.c
index fed1b6e..99b7ec3 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -108,7 +108,7 @@ unsigned int kobjsize(const void *objp)
 	 * The ksize() function is only guaranteed to work for pointers
 	 * returned by kmalloc(). So handle arbitrary pointers here.
 	 */
-	return PAGE_SIZE << compound_order(page);
+	return page_size(page);
 }
 
 /**