libafs: use kthread_run when available

libafs: use kthread_run when available

Use the kthread_run interface on linux to create kernel
threads.  This interface allows all the cpus to schedule
afsd threads, instead of just inheriting the cpu affinity of
the main afsd thread.

Written by Tom Keiser.

Reviewed-on: http://gerrit.openafs.org/7915
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit b440d8ffcbb0ac58ecaa34a9c60fe27a0fc91026)

Change-Id: Id05360309db01ac13b8d887b1ef19596ed86b63a
Reviewed-on: http://gerrit.openafs.org/8604
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>