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.

Change-Id: I4bef6a00f50935b8efa057b42cd2147a22c0c1e6
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>