target/file: walk properly over sg list

This patch changes fileio to use for_each_sg() when walking se_task->task_sg
memory passed into from loopback LLD struct scsi_cmnd scatterlist memory.

This addresses an issue where FILEIO backends with loopback where hitting the
following OOPs with mkfs.ext2:

|kernel BUG at include/linux/scatterlist.h:97!
|invalid opcode: 0000 [#1] PREEMPT SMP
|Modules linked in: sd_mod tcm_loop target_core_stgt scsi_tgt target_core_pscsi target_core_file target_core_iblock target_core_mod configfs scsi_mod
|
|Pid: 671, comm: LIO_fileio Not tainted 3.1.0-rc10+ #139 Bochs Bochs
|EIP: 0060:[<e0afd746>] EFLAGS: 00010202 CPU: 0
|EIP is at fd_do_task+0x396/0x420 [target_core_file]
| [<e0aa7884>] __transport_execute_tasks+0xd4/0x190 [target_core_mod]
| [<e0aa797c>] transport_execute_tasks+0x3c/0xf0 [target_core_mod]
|EIP: [<e0afd746>] fd_do_task+0x396/0x420 [target_core_file] SS:ESP 0068:dea47e90

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: stable@kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

Author: Sebastian Andrzej Siewior

drivers/target/target_core_file.c

@@ -289,9 +289,9 @@ static int fd_do_readv(struct se_task *task)
 		return -ENOMEM;
 	}
 
-	for (i = 0; i < task->task_sg_nents; i++) {
-		iov[i].iov_len = sg[i].length;
-		iov[i].iov_base = sg_virt(&sg[i]);
+	for_each_sg(task->task_sg, sg, task->task_sg_nents, i) {
+		iov[i].iov_len = sg->length;
+		iov[i].iov_base = sg_virt(sg);
 	}
 
 	old_fs = get_fs();
@@ -342,9 +342,9 @@ static int fd_do_writev(struct se_task *task)
 		return -ENOMEM;
 	}
 
-	for (i = 0; i < task->task_sg_nents; i++) {
-		iov[i].iov_len = sg[i].length;
-		iov[i].iov_base = sg_virt(&sg[i]);
+	for_each_sg(task->task_sg, sg, task->task_sg_nents, i) {
+		iov[i].iov_len = sg->length;
+		iov[i].iov_base = sg_virt(sg);
 	}
 
 	old_fs = get_fs();