rtnetlink: change rtnl_stats_dump() return value

By returning 0 (or an error) instead of skb->len,
we allow NLMSG_DONE to be appended to the current
skb at the end of a dump, saving a couple of recvmsg()
system calls.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240502113748.1622637-2-edumazet@google.com


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