LU-13578 test: sleep longer in sanity test_39

LU-13578 test: sleep longer in sanity test_39

In sanity test_39r(), sleep for 2 * atime_diff rather than atime_diff + 1.

Lustre-change: https://review.whamcloud.com/47346
Lustre-commit: be2525ffddb4bf55fde77e97b00d1c349119daed

Test-Parameters: trivial testlist=sanity env=ONLY=39r,ONLY_REPEAT=50
Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: Ied508e12c848f6935d2317fb86bddc5341a6156e
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/48831
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>