selftests/bpf: Xsk selftests - Socket Teardown - SKB, DRV

Adds following tests:

1. AF_XDP SKB mode
   c. Socket Teardown
      Create a Tx and a Rx socket, Tx from one socket, Rx on another.
      Destroy both sockets, then repeat multiple times. Only nopoll mode
      is used

2. AF_XDP DRV/Native mode
   c. Socket Teardown
   * 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/20201207215333.11586-5-weqaar.a.janjua@intel.com