netgraph: ng_device: set curvnet when writing from the cdev

[kevans]

Summary

As noted in the comment, ng_snd_item() doesn't *always* need a vnet
context set -- in particular, it'll often enqueue the item instead of
sending it immediately, and the thread that later dequeues the item will
do the right thing. However, in the case of a node like ng_bridge, we
might be sending immediately and require curvnet to be set as it
transitions through the stack.

The cdev could be made visible for use by another vnet that is not the
ng_device's home, so be sure to use that one rather than the prison
vnet.

Reported by: Quentin Thébault <[e-mail]>
MFC after: 3 days

[kevans]

Already landed in a505b80b2f038409300cd05e2ea95822942404e2, and it's clear nobody is using it in a way that this is urgent to MFC independently