exclude rpc_pipefs from system disk info

What does this MR do?

Exclude rpc_pipefs from system disk info

Are there points in the code the reviewer needs to double check?

Simple change

Why was this MR needed?

As originally mentioned at https://gitlab.com/gitlab-org/gitlab-ce/issues/22808 I have one disk shown with 0 bytes/0 bytes data that is not shown with df and should be excluded from the view.

Does this MR meet the acceptance criteria?

• n/a Changelog entry added
• n/a Documentation created/updated
• n/a API support added
• Tests
  • n/a Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together