staging: erofs: don't check special inode layout

Currently, we will check if inode layout is compression or inline if
the inode is special in fill_inode(). Also set ->i_mapping->a_ops for
it. That is pointless since the both modes won't be set for special
inode when creating EROFS filesystem image. So, let's avoid it.

Signed-off-by: Yue Hu <huyue2@yulong.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>