net/tls: Remove VLA usage

In the quest to remove VLAs from the kernel[1], this replaces the VLA
size with the only possible size used in the code, and adds a mechanism
to double-check future IV sizes.

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Dave Watson <davejwatson@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>