IB/core: Move rdma_is_upper_dev_rcu to header file

In order to validate the route, we need an easy way to check if a
net-device belongs to our RDMA device. Move this helper function
to a header file in order to make this check easier.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Reviewed-by: Haggai Eran <haggaie@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>