Linux: d_alias becomes d_u.d_alias

Linux: d_alias becomes d_u.d_alias

The fields in struct dentry are re-arranged so that d_alias
shares space wth d_rcu inside the d_u union.  Some references
need to change from d_alias to d_u.d_alias.

The kernel change was introduced for 3.19 but was also backported
to the 3.18 stable series in 3.18.1, so this commit is required
for 3.19 and current 3.18 kernels.

Change-Id: I711a5a3a89af6e0055381dfd4474ddca2868bb9c
Reviewed-on: http://gerrit.openafs.org/11642
Reviewed-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-by: Michael Laß <lass@mail.uni-paderborn.de>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>