LU-15661 nodemap: fix map mode value for 'both'

LU-15661 nodemap: fix map mode value for 'both'

The patch that introduced the ability to map project IDs with
nodemap changed the value used for the "map both uid and gid"
case, from 0 to 3.
This poses a problem in case of upgrade from a previous Lustre
version, so re-introduce the value 0 as NODEMAP_MAP_BOTH_LEGACY.

Lustre-change: https://review.whamcloud.com/46870
Lustre-commit: TBD (a774ea01a7ffdf177a0229d9794376bddcb9ab57)

Change-Id: I1f605de9c97faff32411da5052e8782a60645767
Fixes: 491f101042 ("LU-14797 sec: add projid to nodemap")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-on: https://review.whamcloud.com/46869
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>