selftests: xsk: Deflakify STATS_RX_DROPPED test

Fix flaky STATS_RX_DROPPED test. The receiver calls getsockopt after
receiving the last (valid) packet which is not the final packet sent in
the test (valid and invalid packets are sent in alternating fashion with
the final packet being invalid). Since the last packet may or may not
have been dropped already, both outcomes must be allowed.

This issue could also be fixed by making sure the last packet sent is
valid. This alternative is left as an exercise to the reader (or the
benevolent maintainers of this file).

This problem was quite visible on certain setups. On one machine this
failure was observed 50% of the time.

Also, remove a redundant assignment of pkt_stream->nb_pkts. This field
is already initialized by __pkt_stream_alloc.

Fixes: 27e934be ("selftests: xsk: make stat tests not spin on getsockopt")
Signed-off-by: Kal Conley <kal.conley@dectris.com>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/r/20230403120400.31018-1-kal.conley@dectris.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>