mlxsw: spectrum_router: More accurately set offload flag

We currently set the RTNH_F_OFFLOAD flag for all routes using remote
action, but this isn't always correct. If none of the nexthops
associated with a gatewayed route can be offloaded into the device, then
any packet hitting it would be trapped to the CPU and forwarded by the
kernel.

Solve this by pushing the setting of the offload flag to after the route
was programmed into the device, thereby allowing us to take all the
parameters into account.

This change will also help us further in the patchset, when we refresh
routes following the reception of NH_{ADD,DEL} events.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>