LU-18744 tests: fix sanity-sec test_27ab

LU-18744 tests: fix sanity-sec test_27ab

When nodemap is activated in sanity-sec test_27ab, we need to make
sure the default nodemap grants root access, so that clients and
servers can be stopped and restarted.
Also fix an incorrect call to 'lctl nodemap_add_idmap'.

Test-Parameters: trivial
Fixes: e3051ad0f1 ("LU-18109 utils: adding nodemap offset capability")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I0adafae67c7637c616c687590bd01ff12f4d6bf2
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58187
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>