LU-8176 libcfs: Add file_dentry() & file_inode() wrapper

LU-8176 libcfs: Add file_dentry() & file_inode() wrapper

Use file_dentry() & file_inode() wrappers to sync
with upstream.

Signed-off-by: Yang Sheng <yang.sheng@intel.com>
Change-Id: I7e62871789617a07dfe083a60374885099d25d51
Reviewed-on: http://review.whamcloud.com/20576
Tested-by: Jenkins
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>