rcu: Use new RCU_POINTER_INITIALIZER for gcc-style initializations

This commit applies the INIT_RCU_POINTER() macro to all uses of
RCU_POINTER_INITIALIZER() that were all too cleverly creating gcc-style
initializations.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: David Howells <dhowells@redhat.com>