scsi: libcxgbi: simplify task->hdr allocation for mgmt cmds

In case of mgmt cmds, task->hdr is dereferenced after transmitting the
pdu in iscsi_tcp_task_xmit(). To handle this case current code
increments the Tx skb reference count and frees the skb in
cxgbi_cleanup_task(). In some error cases this results in skb leak.

To fix this in case of mgmt cmds, allocate a separate buffer for iSCSI
hdr and free this buffer in cxgbi_cleanup_task().

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