rcu-tasks: Repair RCU Tasks Trace quiescence check

rcu-tasks: Repair RCU Tasks Trace quiescence check

The context-switch-time check for RCU Tasks Trace quiescence expects
current->trc_reader_special.b.need_qs to be zero, and if so, updates
it to TRC_NEED_QS_CHECKED.  This is backwards, because if this value
is zero, there is no RCU Tasks Trace grace period in flight, an thus
no need for a quiescent state.  Instead, when a grace period starts,
this field is set to TRC_NEED_QS.

This commit therefore changes the check from zero to TRC_NEED_QS.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>