devmapper: use 'user_xattr' as mountoption for ext4 - fixes #6189

~~~
># docker pull centos
># docker run -i -t centos /bin/bash

bash-4.1# touch file
bash-4.1# setfattr -n user.attr -v 1 file
setfattr: file: Operation not permitted
~~~

With 'user_xattr' as mount option

~~~
bash-4.1# touch file
bash-4.1# setfattr -n user.attr -v 1 file
bash-4.1# getfattr -d -m. file
>#file: file
user.xattr="1"
~~~
Why isn't this
```go
switch errno {
case syscall.ENODATA:
case syscall.ENOTSUP:
...
```