Update system/xattrs_linux.go

- Add Llistxattr() support
- Additionally cleanup Lgetxattr() and implement it properly
  in accordance with getxattr() syscall behavior.

Signed-off-by: Harshavardhana <fharshav@redhat.com>
Filtering is assumed here to avoid other xattrs other than "user" namespace. But they could be listed and left to the application to ignore. let me change that.