gfs2: Revert "introduce qd_bh_get_or_undo"

The qd_bh_get_or_undo() helper introduced by that commit doesn't improve
the code much, so revert it and clean things up in a more useful way in
the next commit.

This reverts commit 7dbc6ae6.

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