staging/lustre: fix build on s390

As reported by Fengguang:
In file included from drivers/staging/lustre/lustre/obdclass/../include/lustre/lustre_idl.h:99:0,
	    from drivers/staging/lustre/lustre/obdclass/../include/lprocfs_status.h:46,
	    from drivers/staging/lustre/lustre/obdclass/../include/obd_support.h:42,
	    from drivers/staging/lustre/lustre/obdclass/../include/obd_class.h:40,
	    from drivers/staging/lustre/lustre/obdclass/lu_object.c:53:
drivers/staging/lustre/lustre/obdclass/../include/lustre/lustre_user.h:356:10: error: field 'lmd_st' has incomplete type
drivers/staging/lustre/lustre/obdclass/../include/lustre/lustre_user.h:361:10: error: field 'lmd_st' has incomplete type

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>