Linux 3.19: No more f_dentry

Linux 3.19: No more f_dentry

Back in kernel 2.6 .20 struct file lost its f_dentry field
which was replaced by f_path.To ease transition f_dentry
was defined as f_dpath.dentry in the same header.This
define finally gets removed with kernel 3.19.

Keep using f_dentry in the code, but add a configure test
for the presence of f_path and the absence of the f_dentry
macro so we can add it if its missing.

Change - Id:I8e8a7e4d3ddd861018de50af1eb7315e730ad529

Change-Id: I4b05aa3d37f01e0e675c420cbf941d682c49c69c
Reviewed-on: http://gerrit.openafs.org/11646
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>