LU-6142 obdclass: make obd_psdev static

LU-6142 obdclass: make obd_psdev static

obd_psdev is only used in one file, so it can be local to that file.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ib74d78e0e72e054d5f998d54f1476216926b293b
Reviewed-on: https://review.whamcloud.com/39395
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Arshad Hussain <arshad.super@gmail.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>