LINUX: create afs_linux_dget() compat wrapper

LINUX: create afs_linux_dget() compat wrapper

For dentry operations that cover multiple dentry aliases of
a single inode, create a compatibility wrapper to hide differences
between the older dget_locked() and the current dget().

No functional change should be incurred by this commit.

Reviewed-on: https://gerrit.openafs.org/12789
Reviewed-by: Andrew Deason <adeason@dson.org>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 74f4bfc627c836c12bb7c188b86d570d2afdcae8)

Change-Id: Id854e5957547a1370cadb400f7f699c30d861fd1
Reviewed-on: https://gerrit.openafs.org/12847
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>