gfs2: untangle the logic in gfs2_drevalidate

Before this patch, function gfs2_drevalidate was a horrific tangle of
unreadable labels, cases and goto statements. This patch tries to
simplify the logic and make it more readable.

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