net: mctp: pass net into route creation

We may not have a mdev pointer, from which we currently extract the net.

Instead, pass the net directly.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Link: https://patch.msgid.link/20250702-dev-forwarding-v5-10-1468191da8a4@codeconstruct.com.au


Signed-off-by: Paolo Abeni <pabeni@redhat.com>