gfs2: Report errors before withdraw

In gfs2_rgrp_verify and compute_bitstructs, make sure to report errors before
withdrawing the filesystem: otherwise, when we withdraw first and withdraw is
configured to panic, we'll never get to the error reporting.

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