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"
~~~

Docker-DCO-1.1-Signed-off-by: Harshavardhana <fharshav@redhat.com> (github: harshavardhana)