netisr: fix compilation without VIMAGE
- Don't use bare vnet(4) definitions in the KASSERT, they aren't available on a kernel without VIMAGE. Just through MPASS() here. This is more of documenting assertion rather than an assertion that may actually fire on an unmodified kernel.
- V_netisr_enable is different to the rest of V_ prefixed globals. On a kernel without VIMAGE it basically doesn't exist, instead of being present as a single instance.