LU-16492 tests: sanity/398b variable used without assignment

LU-16492 tests: sanity/398b variable used without assignment

This patch initilizes 'before' variable with UNIX
timestamp. Variable 'before' was used without assigning
any value.

Test-Parameters: trivial testlist=sanity env=ONLY=398b
Fixes: b4880f37582a ("LU-15483 tests: Improve test 398b")
Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Change-Id: Iba9e361735272d9c640a115f520ee7c60ac41239
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49687
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>