rcu/exp: Remove redundant full memory barrier at the end of GP

rcu/exp: Remove redundant full memory barrier at the end of GP

A full memory barrier is necessary at the end of the expedited grace
period to order:

1) The grace period completion (pictured by the GP sequence
   number) with all preceding accesses. This pairs with rcu_seq_end()
   performed by the concurrent kworker.

2) The grace period completion and subsequent post-GP update side
   accesses. Pairs again against rcu_seq_end().

This full barrier is already provided by the final sync_exp_work_done()
test, making the subsequent explicit one redundant. Remove it and
improve comments.

Signed-off-by: Frederic Weisbecker <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Reviewed-by: Boqun Feng <[email protected]>
Reviewed-by: Neeraj Upadhyay <[email protected]>