Userspace RCU

In six different spots, the userspace-rcu tests raise this warning:

attention : ignoring return value of ‘write’, declared with attribute warn_unused_result [-Wunused-result]

The simple fix is to wrap each offending write() in an assert() (turns out it's the same write() every time):

userspace-rcu/tests/benchmark/test_urcu_lfq.c, line 385:
            assert(write (1, ".", 1));
userspace-rcu/tests/benchmark/test_urcu_lfs.c, line 469:
            assert(write (1, ".", 1));
userspace-rcu/tests/benchmark/test_urcu_wfcq.c, line 495:
            assert(write (1, ".", 1));
userspace-rcu/tests/benchmark/test_urcu_wfq.c, line 353:
            assert(write (1, ".", 1));
userspace-rcu/tests/benchmark/test_urcu_wfs.c, line 483:
            assert(write (1, ".", 1));
userspace-rcu/tests/benchmark/test_urcu_lfs_rcu.c, line 386:
            assert(write (1, ".", 1));