erofs: limit the level of fs stacking for file-backed mounts

erofs: limit the level of fs stacking for file-backed mounts

[ Upstream commit d53cd891f0e4311889349fff3a784dc552f814b9 ]

Otherwise, it could cause potential kernel stack overflow (e.g., EROFS
mounting itself).

Reviewed-by: Sheng Yong <shengyong1@xiaomi.com>
Fixes: fb176750266a ("erofs: add file-backed mount support")
Reviewed-by: Chao Yu <chao@kernel.org>
Reviewed-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>