gfs2: use page_offset in gfs2_page_mkwrite

Without casting page->index to a guaranteed 64-bit type, the value might be
treated as 32-bit on 32-bit platforms and thus get truncated.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>