[SCSI] libcxgbi: pdu read fixes

Fixed the locking and releasing skb in the case of error in the pdu
read path, and added define iscsi_task_cxgbi_data to access the
private data inside the iscsi_task.

Signed-off-by: Karen Xie <kxie@chelsio.com>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>