af_unix: Use offsetof() instead of sizeof().

The length of the AF_UNIX socket address contains an offset to the member
sun_path of struct sockaddr_un.

Currently, the preceding member is just sun_family, and its type is
sa_family_t and resolved to short.  Therefore, the offset is represented by
sizeof(short).  However, it is not clear and fragile to changes in struct
sockaddr_storage or sockaddr_un.

This commit makes it clear and robust by rewriting sizeof() with
offsetof().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>