rcu: Make rcu_gp_cleanup() write only once to ->gp_flags

At the end of rcu_gp_cleanup(), if another grace period is needed, but
not via rcu_accelerate_cbs(), the ->gp_flags field is written twice,
once when making the new grace-period request, and once when clearing
all other types of requests.  This commit therefore adds an else-clause
to avoid this double write.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>