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

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

testapp_xdp_shared_umem() generates pkt_stream on each xsk from xsk_arr,
where normally xsk_arr[0] gets pkt_streams and xsk_arr[1] have them NULLed.
At the end of the test pkt_stream_restore_default() only releases
xsk_arr[0] which leads to memory leaks.

Release the missing pkt_stream at the end of testapp_xdp_shared_umem()

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-v7-5-39fe486593a3@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>