rcu: add an rcu_dereference_index_check()

The sparse RCU-pointer checking relies on type magic that dereferences
the pointer in question.  This does not work if the pointer is in fact
an array index.  This commit therefore supplies a new RCU API that
omits the sparse checking to continue to support rcu_dereference()
on integers.

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