gfs2: Use gfs2_qd_dispose in gfs2_quota_cleanup

Change gfs2_quota_cleanup() to move the quota data objects to dispose of
on a dispose list and call gfs2_qd_dispose() on that list, like
gfs2_qd_shrink_scan() does, instead of disposing of the quota data
objects directly.

This may look a bit pointless by itself, but it will make more sense in
combination with a fix that follows.

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