LU-17454 nodemap: allow mapping for root

LU-17454 nodemap: allow mapping for root

Allow an id mapping for root, to match what is implemented for regular
users, with the following behavior:
- if admin property is set, root remains root.
- if admin property is not set, the idmap for '0' is taken into
  account.
- if admin property is not set and there is no idmap for '0' and
  deny_unknown property is not set, root is squashed to the squash
  uid/gid.
- if admin property is not set and there is no idmap for '0' and
  deny_unknown property is set, root is blocked.

Note that map_mode remains ignored for root. Also, capabilities are
not dropped for root when mapped, just like it is done for regular
users. If admins want to drop root capabilities, root must be
squashed.

sanity-sec test_15 is updated to test root mapping.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Id2e950b99e3b3ba27179408c647e1f7b7c49e32e
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53870
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>