Linux 3.6: dentry_open API change

Linux 3.6: dentry_open API change

dentry_open now takes a path argument that combines the dentry and
the vfsmount pointers.
Add a configure test and a new compat inline function to keep things
cleaner in the main source file.

Reviewed-on: http://gerrit.openafs.org/7982
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 8766a65e97eb90cb6c97ccd35181c441ece14f8a)

Change-Id: I2c0f59ad9aa6e544a2a613e902933d463f22a5b6
Reviewed-on: http://gerrit.openafs.org/8079
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>