staging: erofs: {dir,inode,super}.c: rectify BUG_ONs

commit 8b987bca upstream.

remove all redundant BUG_ONs, and turn the rest
useful usages to DBG_BUGONs.

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>