net: annotate accesses to dev->gso_max_size

dev->gso_max_size is written under RTNL protection, or when the device is
not yet visible, but is read locklessly.

Add the READ_ONCE()/WRITE_ONCE() pairs, and use netif_set_gso_max_size()
where we can to better document what is going on.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>