rcu: Fundamental facility for 'CPU units sequence reading'

This patch add the fundamental facility used by the following patches, so we
can implement the 'CPU units sequence reading' later.

This helps us avoid losing data when there are too many CPUs and too
small of a buffer, since this new approach allows userspace to read out
the data one CPU at a time.  Thus, if the buffer is not large enough,
userspace will get whatever CPUs fit, and can then issue another read
for the remainder of the data.

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