btrfs: Set file size correctly in file clone

Set src_offset = 0, src_length = 20K, dest_offset = 20K. And the
original filesize of the dest file 'file2' is 30K:

  # ls -l /mnt/file2
  -rw-r--r-- 1 root root 30720 Nov 18 16:42 /mnt/file2

Now clone file1 to file2, the dest file should be 40K, but it
still shows 30K:

  # ls -l /mnt/file2
  -rw-r--r-- 1 root root 30720 Nov 18 16:42 /mnt/file2
Signed-off-by: NLi Zefan <lizf@cn.fujitsu.com>
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/ioctl.c

@@ -1873,8 +1873,8 @@ static noinline long btrfs_ioctl_clone(struct file *file, unsigned long srcfd,
 			 * but shouldn't round up the file size
 			 */
 			endoff = new_key.offset + datal;
-			if (endoff > off+olen)
-				endoff = off+olen;
+			if (endoff > destoff+olen)
+				endoff = destoff+olen;
 			if (endoff > inode->i_size)
 				btrfs_i_size_write(inode, endoff);