gfs2: Minor gfs2_page_add_databufs cleanup

The to parameter of gfs2_page_add_databufs is passed inconsistently:
once as from + len, once as from + len - 1.  Just pass len instead.

In addition, once we're past the end, we can immediately break out of
the loop.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>