ipv4: update comment to document GSO fragmentation cases.

This is a follow-up to commit 9ee6c5dc ("ipv4: allow local
fragmentation in ip_finish_output_gso()"), updating the comment
documenting cases in which fragmentation is needed for egress
GSO packets.

Suggested-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Reviewed-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: Lance Richardson <lrichard@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>