net: Add struct sockaddr_unsized for sockaddr of unknown length

net: Add struct sockaddr_unsized for sockaddr of unknown length

Add flexible sockaddr structure to support addresses longer than the
traditional 14-byte struct sockaddr::sa_data limitation without
requiring the full 128-byte sa_data of struct sockaddr_storage. This
allows the network APIs to pass around a pointer to an object that
isn't lying to the compiler about how big it is, but must be accompanied
by its actual size as an additional parameter.

It's possible we may way to migrate to including the size with the
struct in the future, e.g.:

struct sockaddr_unsized {
	u16 sa_data_len;
	u16 sa_family;
	u8  sa_data[] __counted_by(sa_data_len);
};

Signed-off-by: Kees Cook <kees@kernel.org>
Link: https://patch.msgid.link/20251104002617.2752303-1-kees@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>