Linux 3.11: Adapt to d_count changes

In preparation for upcoming changes in the 3.12 cycle, d_lockref
was introduced late in the 3.11 cycle.  The dentry's d_lock and
d_count are moved to this new structure.  A new d_lock macro makes
the change transparent for locking, but direct users of d_count
must adapt.  A new d_count() helper function is provided and
should now be used.

Use the new d_count() helper function if available, and move
some of the ifdef logic into a helper compatibility function.

Change-Id: I32a21a174d763fb7df8f1e04da3bb7260684571d
Reviewed-on: http://gerrit.openafs.org/10219
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>