Fix locked address interfaces in vmd(8).

Fix locked address interfaces in vmd(8).

Before comparing the amount of bytes read to the size of a packet
struct, make sure the fd being read was actually the packet injection
pipe(2). Locked address interfaces force using the same copy-based
approach used for the internal dhcp service for "local" interfaces
but were accidentally being treated as reads from the pipe(2) and
not the tap.

This broke networking for any locked address interfaces in vmd(8).

Reported by and ok kn@