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

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>
Cc: Stable <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>