erofs: get rid of ROOT_NID()

Let's open code this helper for simplicity.

Signed-off-by: Ferry Meng <mengferry@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20231026021627.23284-1-mengferry@linux.alibaba.com


Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>