sysfs: skip bin_buffer->buffer while reading

After b31ca3f5 ("sysfs: fix deadlock"), bin read() first writes
data to bb->buffer and bounces it to a transient kernel buffer which
is then copied out to userland.  The double bouncing doesn't add
anything.  Let's just use the transient buffer directly.

While at it, rename @temp to @buf for clarity.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/sysfs/bin.c

@@ -72,7 +72,7 @@ read(struct file *file, char __user *userbuf, size_t bytes, loff_t *off)
 	int size = file_inode(file)->i_size;
 	loff_t offs = *off;
 	int count = min_t(size_t, bytes, PAGE_SIZE);
-	char *temp;
+	char *buf;
 
 	if (!bytes)
 		return 0;
@@ -84,23 +84,18 @@ read(struct file *file, char __user *userbuf, size_t bytes, loff_t *off)
 			count = size - offs;
 	}
 
-	temp = kmalloc(count, GFP_KERNEL);
-	if (!temp)
+	buf = kmalloc(count, GFP_KERNEL);
+	if (!buf)
 		return -ENOMEM;
 
 	mutex_lock(&bb->mutex);
+	count = fill_read(file, buf, offs, count);
+	mutex_unlock(&bb->mutex);
 
-	count = fill_read(file, bb->buffer, offs, count);
-	if (count < 0) {
-		mutex_unlock(&bb->mutex);
+	if (count < 0)
 		goto out_free;
-	}
-
-	memcpy(temp, bb->buffer, count);
 
-	mutex_unlock(&bb->mutex);
-
-	if (copy_to_user(userbuf, temp, count)) {
+	if (copy_to_user(userbuf, buf, count)) {
 		count = -EFAULT;
 		goto out_free;
 	}
@@ -110,7 +105,7 @@ read(struct file *file, char __user *userbuf, size_t bytes, loff_t *off)
 	*off = offs + count;
 
  out_free:
-	kfree(temp);
+	kfree(buf);
 	return count;
 }