xfstests: fix btrfs/006 for 10+ devices in SCRATCH_DEV_POOL

xfstests: fix btrfs/006 for 10+ devices in SCRATCH_DEV_POOL

One problem was the output of "uniq -c" which added spaces depending
on the size of the count value (e.g. one space less for 10+ devices).

The second problem was that "btrfs fi show" was doing the same:
"devid %4llu size %s used %s path %s".

Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Reviewed-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>