rcu: fix hang due to race while awaiting a quiescent state

Waiting for a quiescent state happens in two stages: first, we request all
cpus to schedule at least once.  Then, we wait until they do so.

If, between the two stages, a cpu is brought online, then we will request
N cpus to schedule but wait for N+1 to respond.  This of course never happens,
and the system hangs.

Fix by copying the vector which holds the cpus which we signal and wait for;
forcing them to be consistent.  This is safe since newly-added cpus cannot
be accessing any rcu-protected variables before we started signalling.

Fixes random hangs with rcu, mostly seen with 'perf callstack'