ovl: show "userxattr" in the mount data

This was missed when adding the option.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
Reviewed-by: Vivek Goyal <vgoyal@redhat.com>
Fixes: 2d2f2d73 ("ovl: user xattr")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>