LU-16827 obdfilter: Fix "emfperf obdfilter-survey" error

LU-16827 obdfilter: Fix "emfperf obdfilter-survey" error

This patch fixes the definition of the lctl variable. It changes
the logic so that the LCTL value is assigned only when it was
defined earlier.

Fixes: 91a3b286ba ("LU-16827 obdfilter: Fix obdfilter-survery/1a")
Test-Parameters: trivial testlist=obdfilter-survey
Signed-off-by: Vitaliy Kuznetsov <vkuznetsov@ddn.com>
Change-Id: I4dfd7e3d1f78208b33b897d8e6680e59b690014c
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53083
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>