LU-8749 osd-ldiskfs: inherit S_ISGID correctly

LU-8749 osd-ldiskfs: inherit S_ISGID correctly

For remote directory S_ISGID is inherited on agent, not where file
resides, and also the group inherited from parent.

Update sanity 6g to test this.

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: I159b2687ad00fdc7c35f60a18668a015240a1953
Reviewed-on: http://review.whamcloud.com/23329
Tested-by: Jenkins
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>