linux: add read_descriptor_t configure test and ifdef

linux: add read_descriptor_t configure test and ifdef

With linux 2.6.8 the struct read_descriptor_t changed.
Add a configure-test and respective ifdef to deal with that.

Reviewed-on: http://gerrit.openafs.org/4719
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 7133266bafbf238ef921690c40846f3ea2d2fc8b)

Change-Id: Ifdc7c1d3c4162452c1576942b7bfe37ea5014a77
Reviewed-on: http://gerrit.openafs.org/4726
Reviewed-by: Christof Hanke <christof.hanke@rzg.mpg.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>