netdev: let netlink core handle -EMSGSIZE errors

Previous change added -EMSGSIZE handling to af_netlink, we don't
have to hide these errors any longer.

Theoretically the error handling changes from:
 if (err == -EMSGSIZE)
to
 if (err == -EMSGSIZE && skb->len)

everywhere, but in practice it doesn't matter.
All messages fit into NLMSG_GOODSIZE, so overflow of an empty
skb cannot happen.

Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>