LU-9679 lustre: avoid cast of file->private_data

LU-9679 lustre: avoid cast of file->private_data

Instead of
  foo = ((struct seq_file*)file->private_data)->private;
use
  struct seq_file *m = file->private_data;
  foo = m->private;

Many places is lustre use this second style already.
It is much less noisy and prefered for upstream Linux.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I9a7adb102687496f43bab099b1ca584955f040c9
Reviewed-on: https://review.whamcloud.com/36651
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.super@gmail.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>