btrfs: switch write_buf to kernel_write

Instead of playing with the addressing limits.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/btrfs/send.c

@@ -539,33 +539,23 @@ static struct btrfs_path *alloc_path_for_send(void)
 static int write_buf(struct file *filp, const void *buf, u32 len, loff_t *off)
 {
 	int ret;
-	mm_segment_t old_fs;
 	u32 pos = 0;
 
-	old_fs = get_fs();
-	set_fs(KERNEL_DS);
-
 	while (pos < len) {
-		ret = vfs_write(filp, (__force const char __user *)buf + pos,
-				len - pos, off);
+		ret = kernel_write(filp, buf + pos, len - pos, off);
 		/* TODO handle that correctly */
 		/*if (ret == -ERESTARTSYS) {
 			continue;
 		}*/
 		if (ret < 0)
-			goto out;
+			return ret;
 		if (ret == 0) {
-			ret = -EIO;
-			goto out;
+			return -EIO;
 		}
 		pos += ret;
 	}
 
-	ret = 0;
-
-out:
-	set_fs(old_fs);
-	return ret;
+	return 0;
 }
 
 static int tlv_put(struct send_ctx *sctx, u16 attr, const void *data, int len)