linux: update spec requirements

linux: update spec requirements

don't require things which don't exist on el5. do require them
in el6. require kernel-devel if building modules
don't do anything involving the kernel if not building modules
if we can figure out that we don't have 2.4 based on the version,
avoiding the need to depend on kernel-devel just to build an srpm.

Reviewed-on: http://gerrit.openafs.org/6985
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 72f5d88acc752ff447a9e63327c135473326a34d)

Change-Id: Ibcc53eb6a742de2101a054070e9b48607d0bce1d
Reviewed-on: http://gerrit.openafs.org/8787
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>