net/rtnetlink: fix attribute name in nlmsg_size() comments

Use the correct attribute constant names IFLA_GSO_MAX_{SEGS,SIZE}
instead of IFLA_MAX_GSO_{SEGS,SIZE} for the comments int nlmsg_size().

Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>