scsi: libcxgbi: fix skb use after free

skb->data is assigned to task->hdr in cxgbi_conn_alloc_pdu(),
skb gets freed after tx but task->hdr is still dereferenced in
iscsi_tcp_task_xmit() to avoid this call skb_get() after allocating skb
and free the skb in cxgbi_cleanup_task() or before allocating new skb in
cxgbi_conn_alloc_pdu().

Signed-off-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>