LU-4107 build: fix lustre_user.h to C++ compatible

LU-4107 build: fix lustre_user.h to C++ compatible

Fix anonymous union.
Fix warnings
Workaround offsetof due to a bug in g++
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14932

Signed-off-by: Minh Diep <minh.diep@intel.com>
Change-Id: I4c1b8165ab012f4226aabb60009b48063669e271
Reviewed-on: http://review.whamcloud.com/8033
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>