md/bitmap: store bytes in file rather than just in last page.

This number is more generally useful, and bytes-in-last-page is
easily extracted from it.

Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/bitmap.h

@@ -197,7 +197,7 @@ struct bitmap {
 		unsigned long *filemap_attr;	/* attributes associated
 						 * w/ filemap pages */
 		unsigned long file_pages;	/* number of pages in the file*/
-		int last_page_size;		/* bytes in the last page */
+		unsigned long bytes;		/* total bytes in the bitmap */
 	} storage;
 
 	unsigned long flags;