selftests/bpf: Add tests for sock_ops ctx access with same src/dst register

Add selftests to verify SOCK_OPS_GET_SK() and SOCK_OPS_GET_FIELD() correctly
return NULL/zero when dst_reg == src_reg and is_fullsock == 0.

Three subtests are included:
 - get_sk: ctx->sk with same src/dst register (SOCK_OPS_GET_SK)
 - get_field: ctx->snd_cwnd with same src/dst register (SOCK_OPS_GET_FIELD)
 - get_sk_diff_reg: ctx->sk with different src/dst register (baseline)

Each BPF program uses inline asm (__naked) to force specific register
allocation, reads is_fullsock first, then loads the field using the same
(or different) register. The test triggers TCP_NEW_SYN_RECV via a TCP
handshake and checks that the result is NULL/zero when is_fullsock == 0.

Reviewed-by: Sun Jian <sun.jian.kdev@gmail.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20260407022720.162151-3-jiayuan.chen@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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