[NET]: Don't send ICMP_FRAG_NEEDED for GSO packets

Commit 9af3912e

 ("[NET] Move DF check
to ip_forward") added a new check to send ICMP fragmentation needed
for large packets.

Unlike the check in ip_finish_output(), it doesn't check for GSO.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>