md: fix an unsigned compare to allow creation of bitmaps with v1.0 metadata

patch 85bfb4da in mainline.

As page->index is unsigned, this all becomes an unsigned comparison, which
 almost always returns an error.
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/md/bitmap.c

@@ -274,7 +274,7 @@ static int write_sb_page(struct bitmap *bitmap, struct page *page, int wait)
 			if (bitmap->offset < 0) {
 				/* DATA  BITMAP METADATA  */
 				if (bitmap->offset
-				    + page->index * (PAGE_SIZE/512)
+				    + (long)(page->index * (PAGE_SIZE/512))
 				    + size/512 > 0)
 					/* bitmap runs in to metadata */
 					return -EINVAL;