selftests: xsk: Remove end-of-test packet

Get rid of the end-of-test packet and just count the number of packets
received and quit when the expected number as been
received. Simplifies the code.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210825093722.10219-7-magnus.karlsson@gmail.com