net/inet_lro: remove setting skb->ip_summed when not LRO-able

When an SKB cannot be chained to a session, the current code attempts
to "restore" its ip_summed field from lro_mgr->ip_summed. However,
lro_mgr->ip_summed does not hold the original value; in fact, we'd
better not touch skb->ip_summed since it is not modified by the code
in the path leading to a failure to chain it.  Also use a cleaer
comment to the describe the ip_summed field of struct net_lro_mgr.

Issue raised by Or Gerlitz <ogerlitz@voltaire.com>

Signed-off-by: Eli Cohen <eli@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>