target: More core cleanups from AGrover (round 2)

This patch contains the squashed version of second round of target core
cleanups and simplifications and Andy and Co.   It also contains a handful
of fixes to address bugs the original series and other minor cleanups.

Here is the condensed shortlog:

target: Remove unneeded casts to void*
target: Rename get_lun_for_{cmd,tmr} to lookup_{cmd,tmr}_lun
target: Make t_task a member of se_cmd, not a pointer
target: Handle functions returning "-2"
target: Use cmd->se_dev over cmd->se_lun->lun_se_dev
target: Embed qr in struct se_cmd
target: Replace embedded struct se_queue_req with a list_head
target: Rename list_heads that are nodes in struct se_cmd to "*_node"
target: Fold transport_device_setup_cmd() into lookup_{tmr,cmd}_lun()
target: Make t_mem_list and t_mem_list_bidi members of t_task
target: Add comment & cleanup transport_map_sg_to_mem()
target: Remove unneeded checks in transport_free_pages()

(Roland: Fix se_queue_req removal leftovers OOPs)
(nab: Fix transport_lookup_tmr_lun failure case)
(nab: Fix list_empty(&cmd->t_task.t_mem_bidi_list) inversion bugs)

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

drivers/target/tcm_fc/tfc_io.c

@@ -90,15 +90,14 @@ int ft_queue_data_in(struct se_cmd *se_cmd)
 	lport = ep->lp;
 	cmd->seq = lport->tt.seq_start_next(cmd->seq);
 
-	task = se_cmd->t_task;
-	BUG_ON(!task);
+	task = &se_cmd->t_task;
 	remaining = se_cmd->data_length;
 
 	/*
 	 * Setup to use first mem list entry if any.
 	 */
 	if (task->t_tasks_se_num) {
-		mem = list_first_entry(task->t_mem_list,
+		mem = list_first_entry(&task->t_mem_list,
 			 struct se_mem, se_list);
 		mem_len = mem->se_len;
 		mem_off = mem->se_off;
@@ -236,8 +235,7 @@ void ft_recv_write_data(struct ft_cmd *cmd, struct fc_frame *fp)
 	u32 f_ctl;
 	void *buf;
 
-	task = se_cmd->t_task;
-	BUG_ON(!task);
+	task = &se_cmd->t_task;
 
 	fh = fc_frame_header_get(fp);
 	if (!(ntoh24(fh->fh_f_ctl) & FC_FC_REL_OFF))
@@ -315,7 +313,7 @@ void ft_recv_write_data(struct ft_cmd *cmd, struct fc_frame *fp)
 	 * Setup to use first mem list entry if any.
 	 */
 	if (task->t_tasks_se_num) {
-		mem = list_first_entry(task->t_mem_list,
+		mem = list_first_entry(&task->t_mem_list,
 				       struct se_mem, se_list);
 		mem_len = mem->se_len;
 		mem_off = mem->se_off;