quota: Detect loops in quota tree

[ Upstream commit a898cb62 ]

Syzbot has found that when it creates corrupted quota files where the
quota tree contains a loop, we will deadlock when tryling to insert a
dquot. Add loop detection into functions traversing the quota tree.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Sasha Levin <sashal@kernel.org>