LU-8066 obd: harden debugfs handling

LU-8066 obd: harden debugfs handling

While the seq_file private data shouldn't disappear from under
us just in case always test if the private field is set. If
not return -ENODEV for debugfs read and write operations.

Change-Id: I28614994e8ec29f360ab534534a5869b0bf60d4b
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/35575
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.super@gmail.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>