erofs: use poison pointer to replace the hard-coded address

It's safer and cleaner to replace such hard-coded illegal pointer
with poison pointers.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Link: https://lore.kernel.org/r/20230526201459.128169-7-hsiangkao@linux.alibaba.com