selftests/bpf: Xsk selftests - Bi-directional Sockets - SKB, DRV

Adds following tests:

1. AF_XDP SKB mode
   d. Bi-directional Sockets
      Configure sockets as bi-directional tx/rx sockets, sets up fill
      and completion rings on each socket, tx/rx in both directions.
      Only nopoll mode is used

2. AF_XDP DRV/Native mode
   d. Bi-directional Sockets
   * Only copy mode is supported because veth does not currently support
     zero-copy mode

Signed-off-by: Weqaar Janjua <weqaar.a.janjua@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Yonghong Song <yhs@fb.com>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Link: https://lore.kernel.org/bpf/20201207215333.11586-6-weqaar.a.janjua@intel.com