gfs2: Factor out duplicate quota data disposal code

Rename gfs2_qd_dispose() to gfs2_qd_dispose_list().  Move some code
duplicated in gfs2_qd_dispose_list() and gfs2_quota_cleanup() into a
new gfs2_qd_dispose() function.

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