ipv6: Add sockaddr_inet unified address structure

ipv6: Add sockaddr_inet unified address structure

There are cases in networking (e.g. wireguard, sctp) where a union is
used to provide coverage for either IPv4 or IPv6 network addresses,
and they include an embedded "struct sockaddr" as well (for "sa_family"
and raw "sa_data" access). The current struct sockaddr contains a
flexible array, which means these unions should not be further embedded
in other structs because they do not technically have a fixed size (and
are generating warnings for the coming -Wflexible-array-not-at-end flag
addition). But the future changes to make struct sockaddr a fixed size
(i.e. with a 14 byte sa_data member) make the "sa_data" uses with an IPv6
address a potential place for the compiler to get upset about object size
mismatches. Therefore, we need a sockaddr that cleanly provides both an
sa_family member and an appropriately fixed-sized sa_data member that does
not bloat member usage via the potential alternative of sockaddr_storage
to cover both IPv4 and IPv6, to avoid unseemly churn in the affected code
bases.

Introduce sockaddr_inet as a unified structure for holding both IPv4 and
IPv6 addresses (i.e. large enough to accommodate sockaddr_in6).

The structure is defined in linux/in6.h since its max size is sized
based on sockaddr_in6 and provides a more specific alternative to the
generic sockaddr_storage for IPv4 with IPv6 address family handling.

The "sa_family" member doesn't use the sa_family_t type to avoid needing
layer violating header inclusions.

Signed-off-by: Kees Cook <kees@kernel.org>
Link: https://patch.msgid.link/20250722171836.1078436-1-kees@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>