idr: fix obscure bug in allocation path

In sub_alloc(), when bitmap search fails, it goes up one level to
continue search.  This is done by updating the id cursor and searching
the upper level again.  If the cursor was at the end of the upper
level, we need to go further than that.

This wasn't implemented and when that happens the part of the cursor
which indexes into the upper level wraps and sub_alloc() ends up
searching the wrong bitmap.  It allocates id which doesn't match the
actual slot.

This patch fixes this by restarting from the top if the search needs
to go higher than one level.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>