idr: fix RCU lockdep splat in idr_get_next()

Convert to rcu_dereference_raw() given that many callers may have many
different locking models.

Located-by: Miles Lane <miles.lane@gmail.com>
Tested-by: Miles Lane <miles.lane@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>