Btrfs: try to allocate from cluster even at LOOP_NO_EMPTY_SIZE

If we reach LOOP_NO_EMPTY_SIZE, we won't even try to use a cluster that
others might have set up.  Odds are that there won't be one, but if
someone else succeeded in setting it up, we might as well use it, even
if we don't try to set up a cluster again.

Signed-off-by: Alexandre Oliva <oliva@lsd.ic.unicamp.br>
Signed-off-by: Chris Mason <chris.mason@oracle.com>