gfs2: Fix invalid metadata access in punch_hole

In punch_hole(), when the offset lies in the final block for a given
height, there is no hole to punch, but the maximum size check fails to
detect that.  Consequently, punch_hole() will try to punch a hole beyond
the end of the metadata and fail.  Fix the maximum size check.

Signed-off-by: Andrew Price <anprice@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>