LU-18109 nodemap: map back root with offset

LU-18109 nodemap: map back root with offset

When a nodemap is defined with admin=1 and an offset, root should be
mapped back to id 0, independently from the map_mode property.

sanity-sec test_77 is added to exercise this.

Fixes: e3051ad0f1c8 ("LU-18109 utils: adding nodemap offset capability")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I5b220dc90104ceab5c874dfbee41d3e85aefeb5b
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58895
Reviewed-by: Marc Vef <mvef@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>