net: dsa: Remove VLA usage

We avoid 2 VLAs by using a pre-allocated field in dsa_switch. We also
try to avoid dynamic allocation whenever possible (when using fewer than
bits-per-long ports, which is the common case).

Link: http://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com
Link: http://lkml.kernel.org/r/20180505185145.GB32630@lunn.ch


Signed-off-by: Salvatore Mesoraca <s.mesoraca16@gmail.com>
[kees: tweak commit subject and message slightly]
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>