LU-17091 tests: check correct return value in lfs_df

LU-17091 tests: check correct return value in lfs_df

$? is the return value of last command in a pipe.
We should check the return value of first command 'lfs df'
in this case.

Lustre-change: https://review.whamcloud.com/52285
Lustre-commit: TBD (from 8d0b87768034a766a301f945b7a51bf3a3cf0c40)

Signed-off-by: Lei Feng <flei@whamcloud.com>
Test-Parameters: trivial testlist=sanityn
Change-Id: I7daa38f27c878e5195181ed82717cd28ca345dbc
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52291
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>