rcu/nocb: Add checks for offloaded callback processing

This commit is a preparatory patch for offloaded callbacks using the
same ->cblist structure used by non-offloaded callbacks.  It therefore
adds rcu_segcblist_is_offloaded() calls where they will be needed when
!rcu_segcblist_is_enabled() no longer flags the offloaded case.  It also
adds checks in rcu_do_batch() to ensure that there are no missed checks:
Currently, it should not be possible for offloaded execution to reach
rcu_do_batch(), though this will change later in this series.

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