rcu/exp: Remove confusing needless full barrier on task unblock

A full memory barrier in the RCU-PREEMPT task unblock path advertizes
to order the context switch (or rather the accesses prior to
rcu_read_unlock()) with the expedited grace period fastpath.

However the grace period can not complete without the rnp calling into
rcu_report_exp_rnp() with the node locked. This reports the quiescent
state in a fully ordered fashion against updater's accesses thanks to:

1) The READ-SIDE smp_mb__after_unlock_lock() barrier across nodes
   locking while propagating QS up to the root.

2) The UPDATE-SIDE smp_mb__after_unlock_lock() barrier while holding the
   the root rnp to wait/check for the GP completion.

3) The (perhaps redundant given step 1) and 2)) smp_mb() in rcu_seq_end()
   before the grace period completes.

This makes the explicit barrier in this place superfluous. Therefore
remove it as it is confusing.

Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>