net, vrf: Move dstats structure to core

Just move struct pcpu_dstats out of the vrf into the core, and streamline
the field names slightly, so they better align with the {t,l}stats ones.

No functional change otherwise. A conversion of the u64s to u64_stats_t
could be done at a separate point in future. This move is needed as we are
moving the {t,l,d}stats allocation/freeing to the core.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20231114004220.6495-2-daniel@iogearbox.net
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>