netdev-linux: Open AF_PACKET socket only when it is needed.

netdev-linux: Open AF_PACKET socket only when it is needed.

Only a privileged process can open a raw AF_PACKET socket, so netdev-linux
will fail to initialize if run as non-root and you get a cascade of error
messages, like this:

netdev_linux|ERR|failed to create packet socket: Operation not permitted
netdev|ERR|failed to initialize system network device class: Operation not permitted
netdev|ERR|failed to initialize internal network device class: Operation not permitted
netdev|ERR|failed to initialize tap network device class: Operation not permitted

But in fact the AF_PACKET socket is not needed for most operations (only
for sending packets) and it is never needed for testing with the "dummy"
datapath and network device, so we can avoid logging all of these errors
by opening the packet socket only on demand, as this commit does.

Reviewed-by: Simon Horman <horms@verge.net.au>