can: add the missing netlink get_xstats_size callback

This patch adds the missing "get_xstats_size" callback for the
netlink interface, which is required if "fill_xstats" is used,
as pointed out by Patrick McHardy.

Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: David S. Miller <davem@davemloft.net>