erofs: zero out when listxattr is called with no xattr

erofs: zero out when listxattr is called with no xattr

As David reported [1], ENODATA returns when attempting
to modify files by using EROFS as an overlayfs lower layer.

The root cause is that listxattr could return unexpected
-ENODATA by mistake for inodes without xattr. That breaks
listxattr return value convention and it can cause copy
up failure when used with overlayfs.

Resolve by zeroing out if no xattr is found for listxattr.

[1] https://lore.kernel.org/r/CAEvUa7nxnby+rxK-KRMA46=exeOMApkDMAV08AjMkkPnTPV4CQ@mail.gmail.com
Link: https://lore.kernel.org/r/20191201084040.29275-1-hsiangkao@aol.com
Fixes: cadf1ccf1b00 ("staging: erofs: add error handling for xattr submodule")
Cc: <stable@vger.kernel.org> # 4.19+
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>