md: don't write more than is required of the last page of a bitmap

It is possible that real data or metadata follows the bitmap without full page
alignment.

So limit the last write to be only the required number of bytes, rounded up to
the hard sector size of the device.

Signed-off-by: Neil Brown <neilb@suse.de>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/raid/bitmap.h

@@ -232,6 +232,7 @@ struct bitmap {
 	struct page **filemap; /* list of cache pages for the file */
 	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 flags;