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.

Lustre-change: https://review.whamcloud.com/53083
Lustre-commit: 95387e580a639eb9ff0648aecf69d0a4951325ef

Test-Parameters: trivial testlist=obdfilter-survey
Signed-off-by: Vitaliy Kuznetsov <vkuznetsov@ddn.com>
Change-Id: I4dfd7e3d1f78208b33b897d8e6680e59b690014c
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53084
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>