selftests/net: add packet socket packet_snd test

Add regression tests for PF_PACKET transmission using packet_snd.

The TPACKET ring interface has tests for transmission and reception.
This is an initial stab at the same for the send call based interface.

Packets are sent over loopback, then read twice. The entire packet is
read from another packet socket and compared. The packet is also
verified to arrive at a UDP socket for protocol conformance.

The test sends a packet over loopback, testing the following options
(not the full cross-product):

- SOCK_DGRAM
- SOCK_RAW
- vlan tag
- qdisc bypass
- bind() and sendto()
- virtio_net_hdr
- csum offload (NOT actual csum feature, ignored on loopback)
- gso

Besides these basic functionality tests, the test runs from a set
of bounds checks, positive and negative. Running over loopback, which
has dev->min_header_len, it cannot generate variable length hhlen.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>