rcu: Clear request other than RCU_GP_FLAG_INIT at GP end

Once the grace period has ended, any RCU_GP_FLAG_FQS requests are
irrelevant:  The grace period has ended, so there is no longer any
point in forcing quiescent states in order to try to make it end sooner.
This commit therefore causes rcu_gp_cleanup() to clear any bits other
than RCU_GP_FLAG_INIT from ->gp_flags at the end of the grace period.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>