selftests: xsk: Fix missing initialization

Fix missing initialization of the member rx_pkt_nb in the packet
stream. This leads to some tests declaring success too early as the
test thought all packets had already been received.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210922075613.12186-8-magnus.karlsson@gmail.com