LU-18100 pcc: fix the error when get pcc_async_threshold

LU-18100 pcc: fix the error when get pcc_async_threshold

Add "head -n 1" when get llite.*.pcc_async_threshold in the
sanity-pcc/test_102.
Otherwise, it will cause the failure when restore the valule.

Test-Parameters: trivial testlist=sanity-pcc
Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I9618797a09dadbb358601ad602075547c02207c2
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56015
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>