page-types: unsigned cannot be less than 0 in add_page()

If not signed, testing of the read() return value in this function
will not work.
Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/vm/page-types.c

@@ -560,7 +560,7 @@ static void walk_pfn(unsigned long voffset,
 {
 	uint64_t buf[KPAGEFLAGS_BATCH];
 	unsigned long batch;
-	unsigned long pages;
+	long pages;
 	unsigned long i;
 
 	while (count) {