LU-16916 tests: fix client_evicted() not to ignore EOPNOTSUPP

LU-16916 tests: fix client_evicted() not to ignore EOPNOTSUPP

After RHEL 9.x or Ubuntu 22.04 client is evicted, "lfs df" returns
error code 95 (EOPNOTSUPP), which is ignored in check_lfs_df_ret_val()
and then causes client_evicted() to ingore that error.

This patch fixes client_evicted() to check the return value
from "lfs df" directly so as not to ignore EOPNOTSUPP.

Test-Parameters: trivial clientdistro=el9.2 testlist=replay-vbr
Test-Parameters: trivial clientdistro=el8.8 testlist=replay-vbr
Test-Parameters: trivial clientdistro=ubuntu2204 testlist=replay-vbr

Change-Id: I633ae8769fc563b8068f433e2afae29463ac5553
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51667
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>