selftests/bpf: add regression test for ktls+sockmap verdict UAF

selftests/bpf: add regression test for ktls+sockmap verdict UAF

Test the scenario where a socket is inserted into a sockmap with a
BPF_SK_SKB_VERDICT program before TLS RX is configured.  Previously
sk_psock_verdict_data_ready() would call tcp_read_skb() and drain the
receive queue without advancing copied_seq, causing tls_decrypt_sg()
to walk a dangling frag_list pointer (use-after-free).

The test drives the full vulnerable sequence and verifies that after
the fix recv() returns the correct decrypted data.

Signed-off-by: Xingwang Xiang <v3rdant.xiang@gmail.com>
Link: https://patch.msgid.link/20260517145630.20521-3-v3rdant.xiang@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>