LU-17000 utils: Fix check after return from fopen()

LU-17000 utils: Fix check after return from fopen()

Return from fopen() for option 't' (trace) was
checked with a different variable. This patch
fixes this.

Test-Parameters: trivial
CoverityID: 397651 ("Copy-paste error")
Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Change-Id: I04bd207582d421cc3744e7b0f4298e738502edbe
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53686
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>