Linux 3.4: Switch to d_make_root

Linux 3.4: Switch to d_make_root

If d_make_root is available, use it instead of d_alloc_root.
The helper was introduced in 3.3, and d_alloc_root gets removed
in 3.4.

Reviewed-on: http://gerrit.openafs.org/6931
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit f246aa5f84a5233b6525ae43456a8da5b7e92984)

Change-Id: Ic0fccd9f09f811bab6e49639149e41fb71841547
Reviewed-on: http://gerrit.openafs.org/6933
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>