ethtool: Document the general convention for VLAs in kernel space

Various ethtool command structures are declared with zero-length array
at the end which are intended to be variable-length in userland
(relying on lack of compiler bounds checking).  However, in the kernel
the structure and array are always allocated and passed to driver
operations separately.  Make that explicit.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>