[$] Checked-size array parameters in C

There are many possible programmer mistakes that are not caught by the
minimal checks specified by the C language; among those is passing an array
of the wrong size to a function. A recent attempt to add some safety
around array parameters within the crypto layer involved the use of some
clever tricks, but it turns out that clever tricks are unnecessary in this
case. There is an obscure C feature that can cause this checking to
happen, and it is already in use in a few places within the kernel.