LU-9929 nodemap: add default ACL unmapping handling

LU-9929 nodemap: add default ACL unmapping handling

This patch adds default ACL unmapping code to mdt_getxattr
functions so that clients can get a correctly unmapped id.
Also, test_23b is added to sanity-sec.sh to verify this fix.

Change-Id: I6562372c58ca9772f16f7d6b0b98b45ada87971a
Test-Parameters: testlist=sanity-sec
Signed-off-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-on: https://review.whamcloud.com/29010
Tested-by: Jenkins
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Stephan Thiell <sthiell@stanford.edu>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>