btrfs: skip LOOP_NO_EMPTY_SIZE if not clustered allocation

LOOP_NO_EMPTY_SIZE is solely dedicated for clustered allocation. So, we
can skip this stage and give up the allocation.

Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>