netdump: check the support status of the interface

If the interface does not support debugnet(4) we should bail early,
rather than having the user find this out at the time of the panic.
dumpon(8) already expects this return value and will print a helpful
error message.

Reviewed by: cem, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D35180