Btrfs: check for read permission on src file in the clone ioctl

The existing code would have allowed you to clone a file that was
only open for writing
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/ioctl.c

@@ -1480,12 +1480,17 @@ static noinline long btrfs_ioctl_clone(struct file *file, unsigned long srcfd,
 		ret = -EBADF;
 		goto out_drop_write;
 	}
+
 	src = src_file->f_dentry->d_inode;
 
 	ret = -EINVAL;
 	if (src == inode)
 		goto out_fput;
 
+	/* the src must be open for reading */
+	if (!(src_file->f_mode & FMODE_READ))
+		goto out_fput;
+
 	ret = -EISDIR;
 	if (S_ISDIR(src->i_mode) || S_ISDIR(inode->i_mode))
 		goto out_fput;