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: Chris Mason <[e-mail]>

fs/btrfs/ioctl.c

···
 		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;