selftests: net: add PF_PACKET TPACKET v1/v2/v3 selftests

selftests: net: add PF_PACKET TPACKET v1/v2/v3 selftests

This patch adds a simple test case that probes the packet socket's
TPACKET_V1, TPACKET_V2 and TPACKET_V3 behavior regarding mmap(2)'ed
I/O for a small burst of 100 packets. The test currently runs for ...

  TPACKET_V1: RX_RING, TX_RING
  TPACKET_V2: RX_RING, TX_RING
  TPACKET_V3: RX_RING

... and will output on success:

  test: TPACKET_V1 with PACKET_RX_RING .................... 100 pkts (9600 bytes)
  test: TPACKET_V1 with PACKET_TX_RING .................... 100 pkts (9600 bytes)
  test: TPACKET_V2 with PACKET_RX_RING .................... 100 pkts (9600 bytes)
  test: TPACKET_V2 with PACKET_TX_RING .................... 100 pkts (9600 bytes)
  test: TPACKET_V3 with PACKET_RX_RING .................... 100 pkts (9600 bytes)
  OK. All tests passed

Reusable parts of psock_fanout.c have been put into a psock_lib.h
file for common usage. Test case successfully tested on x86_64.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>