net/bpf: Avoid unused "sin_addr_len" warning when CONFIG_CGROUP_BPF is not set

It was reported that there is a compiler warning on the unused variable
"sin_addr_len" in af_inet.c when CONFIG_CGROUP_BPF is not set.
This patch is to address it similar to the ipv6 counterpart
in inet6_getname(). It is to "return sin_addr_len;"
instead of "return sizeof(*sin);".

Fixes: fefba7d1 ("bpf: Propagate modified uaddrlen from cgroup sockaddr programs")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/bpf/20231013185702.3993710-1-martin.lau@linux.dev

Closes: https://lore.kernel.org/bpf/20231013114007.2fb09691@canb.auug.org.au/

net/ipv4/af_inet.c

@@ -814,7 +814,7 @@ int inet_getname(struct socket *sock, struct sockaddr *uaddr,
 	}
 	release_sock(sk);
 	memset(sin->sin_zero, 0, sizeof(sin->sin_zero));
-	return sizeof(*sin);
+	return sin_addr_len;
 }
 EXPORT_SYMBOL(inet_getname);