rcu: Avoid losing ->need_future_gp[] values due to GP start/end races

The rcu_cbs_completed() function provides the value of ->completed
at which new callbacks can safely be invoked.  This is recorded in
two-element ->need_future_gp[] arrays in the rcu_node structure, and
the elements of these arrays corresponding to the just-completed grace
period are zeroed at the end of that grace period.  However, the
rcu_cbs_completed() function can return the current ->completed value
plus either one or two, so it is possible for the corresponding
->need_future_gp[] entry to be cleared just after it was set, thus
losing a request for a future grace period.

This commit avoids this race by expanding ->need_future_gp[] to four
elements.

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