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.

Change-Id: I69eb852d168bd85e9aa7ec075013c0346207dbcf
Reviewed-on: http://gerrit.openafs.org/7915
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>