uml: use simple_write_to_buffer()

Simplify write file operation for mmapper by using
simple_write_to_buffer().

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/drivers/mmapper_kern.c

@@ -37,13 +37,7 @@ static ssize_t mmapper_write(struct file *file, const char __user *buf,
 	if (*ppos > mmapper_size)
 		return -EINVAL;
 
-	if (count > mmapper_size - *ppos)
-		count = mmapper_size - *ppos;
-
-	if (copy_from_user(&v_buf[*ppos], buf, count))
-		return -EFAULT;
-
-	return count;
+	return simple_write_to_buffer(v_buf, mmapper_size, ppos, buf, count);
 }
 
 static long mmapper_ioctl(struct file *file, unsigned int cmd, unsigned long arg)