iplink: add gro_max_size attribute handling

iplink: add gro_max_size attribute handling

Add the ability to display or change the gro_max_size attribute.

ip link set dev eth1 gro_max_size 60000
ip -d link show eth1
5: eth1: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 9198 qdisc mq master eth0 state UP mode DEFAULT group default qlen 1000
    link/ether bc:ae:c5:39:69:66 brd ff:ff:ff:ff:ff:ff promiscuity 0 minmtu 46 maxmtu 9600
    <...> gro_max_size 60000

Signed-off-by: Coco Li <lixiaoyan@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David Ahern <dsahern@kernel.org>