Treat Linux 4 (and greater) as Linux 2.6/3

Treat Linux 4 (and greater) as Linux 2.6/3

In an age where Linux version numbers are determined by Google+ polls,
it’s clear that they aren’t going to be very useful for marking major
API compatibility boundaries like they were in the days of 2.2/2.4.

Reviewed-on: http://gerrit.openafs.org/11755
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit a5b091e1ec69d4a43d6f1b1efc93134ef7ed2167)

Change-Id: I5b0da6b43e3cbf5d9a6fa883a09deccb359e53e9
Reviewed-on: http://gerrit.openafs.org/11760
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>