selftests/xsk: Introduce default Rx pkt stream

In order to prepare xdpxceiver for physical device testing, let us
introduce default Rx pkt stream. Reason for doing it is that physical
device testing will use a UMEM with a doubled size where half of it will
be used by Tx and other half by Rx. This means that pkt addresses will
differ for Tx and Rx streams. Rx thread will initialize the
xsk_umem_info::base_addr that is added here so that pkt_set(), when
working on Rx UMEM will add this offset and second half of UMEM space
will be used. Note that currently base_addr is 0 on both sides. Future
commit will do the mentioned initialization.

Previously, veth based testing worked on separate UMEMs, so single
default stream was fine.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20220901114813.16275-3-maciej.fijalkowski@intel.com