rdma_rxe: Ensure rdma_rxe init occurs at correct time

There is a problem when CONFIG_RDMA_RXE=y and CONFIG_IPV6=y. This
results in the rdma_rxe initialization occurring before the IPv6
services are ready. This patch delays the initialization of rdma_rxe
until after the IPv6 services are ready. This fix is based on one
proposed by Logan Gunthorpe on a much older code base.

Signed-off-by: Stephen Bates <sbates@raithlin.com>
Reviewed-by: Yonatan Cohen <yonatanc@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>