RDMA/core: Squash a single user static function

To reduce dependencies in IFF_LOOPBACK in route and neighbour resolution
steps, squash the static function to its single caller and simplify the
code.
Until now, network field was set even when neighbour resolution failed.
With this change, dev_addr output fields are valid only when resolution
is successful.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Vlad Dumitrescu <vdumitrescu@nvidia.com>
Signed-off-by: Edward Srouji <edwards@nvidia.com>
Link: https://patch.msgid.link/20250916111103.84069-2-edwards@nvidia.com


Signed-off-by: Leon Romanovsky <leon@kernel.org>