staging: lustre: fix sparse error: incompatible types in comparison expression

Comparing two user space addresses to avoid sparse error:

drivers/staging//lustre/lnet/selftest/conrpc.c:490:30: error:
incompatible types in comparison expression (different address spaces)

Signed-off-by: Rui Teng <rui.teng@linux.vnet.ibm.com>
Acked-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>