LU-3671 mdd: sync perm for dir and perm reduction only

To mitigate performance impact for sync permission, do it for dir
and when permission is reduced only.

Remove extra sync permission check in mdd_xattr_set() because ACL
change is all handled in mdd_acl_set().

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: I64fc720a380aafdaf858d83fbb85f29a1063e07f
Reviewed-on: http://review.whamcloud.com/7257
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>