hpfs: deadlock and race in directory lseek()

For one thing, there's an ABBA deadlock on hpfs fs-wide lock and i_mutex
in hpfs_dir_lseek() - there's a lot of methods that grab the former with
the caller already holding the latter, so it must take i_mutex first.

For another, locking the damn thing, carefully validating the offset,
then dropping locks and assigning the offset is obviously racy.

Moreover, we _must_ do hpfs_add_pos(), or the machinery in dnode.c
won't modify the sucker on B-tree surgeries.
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

fs/hpfs/dir.c

@@ -33,25 +33,27 @@ static loff_t hpfs_dir_lseek(struct file *filp, loff_t off, int whence)
 	if (whence == SEEK_DATA || whence == SEEK_HOLE)
 		return -EINVAL;
 
+	mutex_lock(&i->i_mutex);
 	hpfs_lock(s);
 
 	/*printk("dir lseek\n");*/
 	if (new_off == 0 || new_off == 1 || new_off == 11 || new_off == 12 || new_off == 13) goto ok;
-	mutex_lock(&i->i_mutex);
 	pos = ((loff_t) hpfs_de_as_down_as_possible(s, hpfs_inode->i_dno) << 4) + 1;
 	while (pos != new_off) {
 		if (map_pos_dirent(i, &pos, &qbh)) hpfs_brelse4(&qbh);
 		else goto fail;
 		if (pos == 12) goto fail;
 	}
-	mutex_unlock(&i->i_mutex);
+	hpfs_add_pos(i, &filp->f_pos);
 ok:
+	filp->f_pos = new_off;
 	hpfs_unlock(s);
-	return filp->f_pos = new_off;
-fail:
 	mutex_unlock(&i->i_mutex);
+	return new_off;
+fail:
 	/*printk("illegal lseek: %016llx\n", new_off);*/
 	hpfs_unlock(s);
+	mutex_unlock(&i->i_mutex);
 	return -ESPIPE;
 }