LINUX: make d_automount work properly on rhel5 kernels

LINUX: make d_automount work properly on rhel5 kernels

Recent centos/rhel 5 kernels (2.6.18-308.*) started providing the
d_automount operation, but renamed the DCACHE_NEED_AUTOMOUNT flag to
DMANAGED_AUTOMOUNT.

Reviewed-on: http://gerrit.openafs.org/8006
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 5daa08ea263b2a82b4a85abd61d72d4d1f48bbb6)

Change-Id: I1ebc2a3cab52ff7294b03e1e82680a99d62013fc
Reviewed-on: http://gerrit.openafs.org/8790
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>