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.

Change-Id: I56e0e88eb178573c3eb280d5a5a01d8b8a20a363
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>