selftests/landlock: Fix TCP bind(AF_UNSPEC) test case

selftests/landlock: Fix TCP bind(AF_UNSPEC) test case

The nominal error code for bind(AF_UNSPEC) on an IPv6 socket
is -EAFNOSUPPORT, not -EINVAL. -EINVAL is only returned when
the supplied address struct is too short, which happens to be
the case in current selftests because they treat AF_UNSPEC
like IPv4 sockets do: as an alias for AF_INET (which is a
16-byte struct instead of the 24 bytes required by IPv6
sockets).

Make the union large enough for any address (by adding struct
sockaddr_storage to the union), and make AF_UNSPEC addresses
large enough for any family.

Test for -EAFNOSUPPORT instead, and add a dedicated test case
for truncated inputs with -EINVAL.

Fixes: a549d055a22e ("selftests/landlock: Add network tests")
Signed-off-by: Matthieu Buffet <matthieu@buffet.re>
Link: https://lore.kernel.org/r/20251027190726.626244-2-matthieu@buffet.re
Signed-off-by: Mickaël Salaün <mic@digikod.net>