LU-6215 llite: file->f_dentry no longer exist

LU-6215 llite: file->f_dentry no longer exist

In newer kernels file->f_dentry no long exist.
Convert the recent changes that used file->f_dentry
to use the proper file_path.dentry. This time
add a checkpatch warning so this can be avoided
in the future.

Change-Id: I5b0c54f3fd80d548c5fa527a3edf95ef03f97122
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: http://review.whamcloud.com/16119
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: frank zago <fzago@cray.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>