call_rcu: use cpu affinity for per-cpu call_rcu threads

I played a bit with the call_rcu() implementation alongside with my
rbtree tests, and noticed the following:

If I use per-cpu call_rcu threads with URCU_CALL_RCU_RT flag, with one
updater thread only for my rbtree (no reader), I get 38365 updates/s.
If I add cpu affinity to these per-cpu call_rcu threads (I have prepared
a patch that does this), it jumps to 54219 updates/s.  So it looks like
keeping per-cpu affinity for the call_rcu thread is a good thing.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>