Currently we use type u64 for byte counts, which can very quickly
exceed 2^32, and unsigned long for packet counts, which do not.  But
it can still take only 20-something minutes to send or receive 2^32
packets, and not all tools properly handle overflow even if they
sample more often than this.

The MAC statistics are all updated synchronously, so it costs very
little to make them all 64-bit regardless of native word size.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>