LinuxKPI: seq_file add "private" versions.

Add __seq_open_private() and seq_release_private() needed by iwlwifi
debugfs support.

Sponsored by: The FreeBSD Foundation
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D37089

(cherry picked from commit b5a81075903b3d97265151960b731210c0e80244)