selftests: drv-net: xdp: make the XDP qstats tests less flaky

selftests: drv-net: xdp: make the XDP qstats tests less flaky

The XDP qstats tests send 2k packets over a single socket.
Looks like when netdev CI is busy running those tests in QEMU
occasionally flakes. The target doesn't get to run at all
before all 2000 packets are sent.

Lower the number of packets to 1000 and reopen the socket
every 50 packets, to give RSS a chance to spread the packets
to multiple queues.

For the netdev CI testing either lowering the count or using
multiple sockets is enough, but let's do both for extra resiliency.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20251113152703.3819756-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>