gfs2: Fix slab-use-after-free in qd_put

Commit a475c5d ("gfs2: Free quota data objects synchronously")
started freeing quota data objects during filesystem shutdown instead of
putting them back onto the LRU list, but it failed to remove these
objects from the LRU list, causing LRU list corruption.  This caused
use-after-free when the shrinker (gfs2_qd_shrink_scan) tried to access
already-freed objects on the LRU list.

Fix this by removing qd objects from the LRU list before freeing them in
qd_put().

Initial fix from Deepanshu Kartikey <kartikey406@gmail.com>.

Fixes: a475c5d ("gfs2: Free quota data objects synchronously")
Reported-by: syzbot+046b605f01802054bff0@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=046b605f01802054bff0
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

Author: Andreas Gruenbacher

fs/gfs2/quota.c

@@ -334,6 +334,7 @@ static void qd_put(struct gfs2_quota_data *qd)
 		lockref_mark_dead(&qd->qd_lockref);
 		spin_unlock(&qd->qd_lockref.lock);
 
+		list_lru_del_obj(&gfs2_qd_lru, &qd->qd_lru);
 		gfs2_qd_dispose(qd);
 		return;
 	}