selftests/bpf: test_xsk: fix memory leak in testapp_stats_rx_dropped()

selftests/bpf: test_xsk: fix memory leak in testapp_stats_rx_dropped()

testapp_stats_rx_dropped() generates pkt_stream twice. The last
generated is released by pkt_stream_restore_default() at the end of the
test but we lose the pointer of the first pkt_stream.

Release the 'middle' pkt_stream when it's getting replaced to prevent
memory leaks.

Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Link: https://lore.kernel.org/r/20251031-xsk-v7-4-39fe486593a3@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>