LU-6215 kernel: use f_path for inode dentry

LU-6215 kernel: use f_path for inode dentry

Starting with linux kernel 2.6.17 the f_dentry field of
struct inode was moved to f_path from the top level. To
prevent breakage a backwards compatible macro was provided.
The linux kernel 3.18 removed that  macro which causes
lustre to fail to compile. This patch migrates the code
over to using the proper field.

Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Change-Id: I1497fa6667dfe1e72db0a6c7e2da78af9acdb1ee
Reviewed-on: http://review.whamcloud.com/14598
Tested-by: Jenkins
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>