vrf: Make pcpu_dstats update functions available to other modules.

Currently vrf is the only module that uses NETDEV_PCPU_STAT_DSTATS.
In order to make this kind of statistics available to other modules,
we need to define the update functions in netdevice.h.

Therefore, let's define dev_dstats_*() functions for RX and TX packet
updates (packets, bytes and drops). Use these new functions in vrf.c
instead of vrf_rx_stats() and the other manual counter updates.

While there, update the type of the "len" variables to "unsigned int",
so that there're aligned with both skb->len and the new dstats update
functions.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Link: https://patch.msgid.link/d7a552ee382c79f4854e7fcc224cf176cd21150d.1733313925.git.gnault@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>