rcu/nocb: Allow lockless use of rcu_segcblist_restempty()

Currently, rcu_segcblist_restempty() assumes that the callback list
is not being changed by other CPUs, but upcoming changes will require
it to operate locklessly.  This commit therefore adds the needed
READ_ONCE() calls, along with the WRITE_ONCE() calls when updating
the callback list.

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