gfs2: Set .migrate_folio in gfs2_{rgrp,meta}_aops

[ Upstream commit 5c8f12cf ]

Clears up the warning added in 7ee36472 ("migrate: Remove call to
->writepage") that occurs in various xfstests, causing "something found
in dmesg" failures.

[  341.136573] gfs2_meta_aops does not implement migrate_folio
[  341.136953] WARNING: CPU: 1 PID: 36 at mm/migrate.c:944 move_to_new_folio+0x2f8/0x300

Signed-off-by: Andrew Price <anprice@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>