sctp: set the skb->ip_summed correctly when sending over loopback.

Loopback used to clobber the ip_summed filed which sctp then used
to figure out if it needed to do checksumming or not.  Now that
loopback doesn't do that any more, sctp needs to set the ip_summed
field correctly.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>