target: Eliminate usage of struct se_mem

Both backstores and fabrics use arrays of struct scatterlist to describe
data buffers. However TCM used struct se_mems, basically a linked list
of scatterlist entries. We are able to simplify the code by eliminating
this intermediate data structure and just using struct scatterlist[]
throughout.

Also, moved attachment of task to cmd out of transport_generic_get_task
and into allocate_control_task and allocate_data_tasks. The reasoning
is that it's nonintuitive that get_task should automatically add it to
the cmd's task list -- it should just return an allocated, initialized
task. That's all it should do, based on the function's name, so either the
function shouldn't do it, or the name should change to encapsulate the
entire essence of what it does.

(nab: Fix compile warnings in tcm_fc, and make transport_kmap_first_data_page
 honor sg->offset for SGLs from contigious memory with TCM_Loop, and
 fix control se_cmd descriptor memory leak)

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

drivers/target/loopback/tcm_loop.c

@@ -175,10 +175,7 @@ static int tcm_loop_new_cmd_map(struct se_cmd *se_cmd)
 		sgl_bidi_count = sdb->table.nents;
 	}
 
-	/*
-	 * Map the SG memory into struct se_mem->page linked list using the same
-	 * physical memory at sg->page_link.
-	 */
+	/* Tell the core about our preallocated memory */
 	ret = transport_generic_map_mem_to_cmd(se_cmd, scsi_sglist(sc),
 			scsi_sg_count(sc), sgl_bidi, sgl_bidi_count);
 	if (ret < 0)

drivers/target/target_core_iblock.c

@@ -634,7 +634,7 @@ static int iblock_map_task_SG(struct se_task *task)
 	hbio = tbio = bio;
 	/*
 	 * Use fs/bio.c:bio_add_pages() to setup the bio_vec maplist
-	 * from TCM struct se_mem -> task->task_sg -> struct scatterlist memory.
+	 * from task->task_sg -> struct scatterlist memory.
 	 */
 	for_each_sg(task->task_sg, sg, task->task_sg_num, i) {
 		DEBUG_IBLOCK("task: %p bio: %p Calling bio_add_page(): page:"

drivers/target/target_core_pscsi.c

@@ -1097,7 +1097,7 @@ static int __pscsi_map_task_SG(
 		return 0;
 	/*
 	 * For SCF_SCSI_DATA_SG_IO_CDB, Use fs/bio.c:bio_add_page() to setup
-	 * the bio_vec maplist from TC< struct se_mem -> task->task_sg ->
+	 * the bio_vec maplist from task->task_sg ->
 	 * struct scatterlist memory.  The struct se_task->task_sg[] currently needs
 	 * to be attached to struct bios for submission to Linux/SCSI using
 	 * struct request to struct scsi_device->request_queue.