[SCSI] cxgb3i: remove use of skb->sp

The cxgb3i was using skb->sp pointer for some internal book-keeping
which is not related to the secure path. Changed it to use skb->cb[]
instead.

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Karen Xie <kxie@chelsio.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>