net: Use sockaddr_storage for getsockopt(SO_PEERNAME).

Commit df8fc4e9 ("kbuild: Enable -fstrict-flex-arrays=3") started
applying strict rules to standard string functions.

It does not work well with conventional socket code around each protocol-
specific sockaddr_XXX struct, which is cast from sockaddr_storage and has
a bigger size than fortified functions expect.  See these commits:

 commit 06d4c8a8 ("af_unix: Fix fortify_panic() in unix_bind_bsd().")
 commit ecb4534b ("af_unix: Terminate sun_path when bind()ing pathname socket.")
 commit a0ade840

 ("af_packet: Fix warning of fortified memcpy() in packet_getname().")

We must cast the protocol-specific address back to sockaddr_storage
to call such functions.

However, in the case of getsockaddr(SO_PEERNAME), the rationale is a bit
unclear as the buffer is defined by char[128] which is the same size as
sockaddr_storage.

Let's use sockaddr_storage explicitly.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>