net: stats: Make byte counters 64-bit

In high throughput tests it's fairly easy to overflow the current 32-bit
byte counters in net statistics (it's just over 4 GB of data).
Therefore, make the byte counters 64-bit to prevent overflows.
Rearrange some fields to avoid padding.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>