Reduce PACKET_RX_RING memory usage
Reduce PACKET_RX_RING memory usage

Previous memory allocation was excessive (80 MB). Changed
it to use 2 MB instead. There is no drop in perfomance due
to this change:

ab -n 100 -c 10 http://server/latin10m.txt  ==> 10 MB file
80 MB: 178 MB/s
 2 MB: 181 MB/s

PiperOrigin-RevId: 238321594

google/netstack