selftests/bpf: Fix memory leaks in tests

selftests/bpf: Fix memory leaks in tests

Fix trivial memory leaks detected by userspace ASAN:
  - htab_update: free value buffer in test_reenter_update cleanup
  - test_xsk: inline pkt_stream_replace() in testapp_stats_rx_full()
    and testapp_stats_fill_empty()
  - testing_helpers: free buffer allocated by getline() in
    parse_test_list_file

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260223190736.649171-11-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>