mm: get 7% more pages in a pagevec

We don't have to use an entire 'long' for the number of elements in the
pagevec; we know it's a number between 0 and 14 (now 15).  So we can
store it in a char, and then the bool packs next to it and we still have
two or six bytes of padding for more elements in the header.  That gives
us space to cram in an extra page.

Link: http://lkml.kernel.org/r/20171206022521.GM26021@bombadil.infradead.org
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/pagevec.h

@@ -9,14 +9,14 @@
 #ifndef _LINUX_PAGEVEC_H
 #define _LINUX_PAGEVEC_H
 
-/* 14 pointers + two long's align the pagevec structure to a power of two */
-#define PAGEVEC_SIZE	14
+/* 15 pointers + header align the pagevec structure to a power of two */
+#define PAGEVEC_SIZE	15
 
 struct page;
 struct address_space;
 
 struct pagevec {
-	unsigned long nr;
+	unsigned char nr;
 	bool percpu_pvec_drained;
 	struct page *pages[PAGEVEC_SIZE];
 };