bpf: Don't check sk_fullsock() in bpf_skc_to_unix_sock().

AF_UNIX does not use TCP_NEW_SYN_RECV nor TCP_TIME_WAIT and
checking sk->sk_family is sufficient.

Let's remove sk_fullsock() and use sk_is_unix() in
bpf_skc_to_unix_sock().

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20260203213442.682838-3-kuniyu@google.com

Author: q2ven

net/core/filter.c

@@ -12021,7 +12021,7 @@ BPF_CALL_1(bpf_skc_to_unix_sock, struct sock *, sk)
 	 * trigger an explicit type generation here.
 	 */
 	BTF_TYPE_EMIT(struct unix_sock);
-	if (sk && sk_fullsock(sk) && sk->sk_family == AF_UNIX)
+	if (sk && sk_is_unix(sk))
 		return (unsigned long)sk;
 
 	return (unsigned long)NULL;