Size of the attribute IFLA_PHYS_PORT_NAME was missing.

Fixes: db24a904 ("net: add support for phys_port_name")
CC: David Ahern <dsahern@gmail.com>
Signed-off-by: NNicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: NDavid Ahern <dsahern@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>