dpif-netdev: Avoid variable length array on MSVC.

MSVC does not like variable length array either.

This patch treats the following error:

lib/dpif-netdev.c(2272) : error C2057: expected constant expression
lib/dpif-netdev.c(2272) : error C2466: cannot allocate an array of constant size 0
lib/dpif-netdev.c(2272) : error C2133: 'batches' : unknown size
lib/dpif-netdev.c(2273) : error C2057: expected constant expression
lib/dpif-netdev.c(2273) : error C2466: cannot allocate an array of constant size 0
lib/dpif-netdev.c(2273) : error C2133: 'mfs' : unknown size
lib/dpif-netdev.c(2274) : error C2057: expected constant expression
lib/dpif-netdev.c(2274) : error C2466: cannot allocate an array of constant size 0
lib/dpif-netdev.c(2274) : error C2133: 'rules' : unknown size
lib/dpif-netdev.c(2363) : warning C4034: sizeof returns 0
lib/dpif-netdev.c(2381) : error C2057: expected constant expression
lib/dpif-netdev.c(2381) : error C2466: cannot allocate an array of constant size 0
lib/dpif-netdev.c(2381) : error C2133: 'keys' : unknown size
make[2]: *** [lib/dpif-netdev.lo] Error 1

Signed-off-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>