target: Simplify fabric sense data length handling

Every fabric driver has to supply a se_tfo->set_fabric_sense_len()
method, just so iSCSI can return an offset of 2.  However, every fabric
driver is already allocating a sense buffer and passing it into the
target core, either via transport_init_se_cmd() or target_submit_cmd().

So instead of having iSCSI pass the start of its sense buffer into the
core and then later tell the core to skip the first 2 bytes, it seems
easier for iSCSI just to do the offset of 2 when it passes the sense
buffer into the core.  Then we can drop the se_tfo->set_fabric_sense_len()
everywhere, and just add a couple of lines of code to iSCSI to set the
sense data length to the beginning of the buffer right before it sends
it over the network.

(nab: Remove .set_fabric_sense_len usage from tcm_qla2xxx_npiv_ops +
      change transport_get_sense_buffer to follow v3.6-rc6 code w/o
      ->set_fabric_sense_len usage)

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/infiniband/ulp/srpt/ib_srpt.c

@@ -3564,11 +3564,6 @@ static int srpt_get_tcm_cmd_state(struct se_cmd *se_cmd)
 	return srpt_get_cmd_state(ioctx);
 }
 
-static u16 srpt_set_fabric_sense_len(struct se_cmd *cmd, u32 sense_length)
-{
-	return 0;
-}
-
 /**
  * srpt_parse_i_port_id() - Parse an initiator port ID.
  * @name: ASCII representation of a 128-bit initiator port ID.
@@ -3948,7 +3943,6 @@ static struct target_core_fabric_ops srpt_template = {
 	.queue_data_in			= srpt_queue_response,
 	.queue_status			= srpt_queue_status,
 	.queue_tm_rsp			= srpt_queue_response,
-	.set_fabric_sense_len		= srpt_set_fabric_sense_len,
 	/*
 	 * Setup function pointers for generic logic in
 	 * target_core_fabric_configfs.c