ovs-thread: Do not always end quiescent state in ovs_thread_create().

ovs-thread: Do not always end quiescent state in ovs_thread_create().

A new thread must be started in a non quiescent state.  There is a call
to ovsrcu_quiesce_end() in ovsthread_wrapper(), to enforce this.

ovs_thread_create(), instead, is executed in the parent thread. It must
call ovsrcu_quiesce_end() on its first invocation, to put the main
thread in a non quiescent state.  On every other invocation, it doesn't
make sense to alter the calling thread state, so this commits wraps the
call to ovsrcu_quiesce_end() in an ovsthread_once construct.

This fixes a bug in ovs-rcu where the first call in the process to
ovsrcu_quiesce_start() will not be honored, because the calling thread
will need to create the 'urcu' thread (and creating a thread will
wrongly end its quiescent state).

ovsrcu_quiesce_start()
  ovs_rcu_quiesced()
    if (ovsthread_once_start(&once)) {
        ovs_thread_create("urcu") /*This will end the quiescent state*/
    }

This bug affects in particular ovs-vswitchd with DPDK.
In the DPDK case the first threads created are "vhost_thread" and
"dpdk_watchdog".  If dpdk_watchdog is the first to call
ovsrcu_quiesce_start() (via xsleep()), the call is not honored and
the RCU grace period lasts at least for DPDK_PORT_WATCHDOG_INTERVAL
(5s on current master).  If vhost_thread, on the other hand, is the
first to call ovsrcu_quiesce_start(), the call is not honored and the
RCU grace period lasts undefinitely, because no more calls to
ovsrcu_quiesce_start() are issued from vhost_thread.

For some reason (it's a race condition after all), on current master,
dpdk_watchdog will always be the first to call ovsrcu_quiesce_start(),
but with the upcoming DPDK database configuration changes, sometimes
vhost_thread will issue the first call to ovsrcu_quiesce_start().

Sample ovs-vswitchd.log:

2016-03-23T22:34:28.532Z|00004|ovs_rcu(urcu3)|WARN|blocked 8000 ms
waiting for vhost_thread2 to quiesce
2016-03-23T22:34:30.501Z|00118|ovs_rcu|WARN|blocked 8000 ms waiting for
vhost_thread2 to quiesce
2016-03-23T22:34:36.532Z|00005|ovs_rcu(urcu3)|WARN|blocked 16000 ms
waiting for vhost_thread2 to quiesce
2016-03-23T22:34:38.501Z|00119|ovs_rcu|WARN|blocked 16000 ms waiting for
vhost_thread2 to quiesce

The commit also adds a test for the ovs-rcu module to make sure that:
* A new thread is started in a non quiescent state.
* The first call to ovsrcu_quiesce_start() is honored.
* When a process becomes multithreaded the main thread is put in an
  active state

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Ben Pfaff <blp@ovn.org>