cluster/tier: readdirp to cold tier only

cluster/tier: readdirp to cold tier only

It is possible a file would get migrated in the middle
of a readdir operation. If there are four subvolumes A,B,C,D,
and if readdir reads them in order and reaches subvol B,
then, if a file is moved from D to A, it will not be included
in the readdir output.

This phenonema has pre-existed in DHT migration but is more
apparent in tiering.

When a file is moved off the hashed subvolume a T file is created.
For tiering, we will make the cold subvolume the hashed subvolume.
This will ensure the creation of a T file. Readdir will not skip T
files in the tier translator.

Making the cold subvolume the hashed subvolume ensures the T
files created on promotions or creates will be less likely to
fill the volume.

Creates still put the data on the hot subvolume.

Change-Id: Ifde557d3d0e94a4570ca9f115adee3db2ee75407
BUG:  1281598
Signed-off-by: Dan Lambright <dlambrig@redhat.com>
Reviewed-on: http://review.gluster.org/12530
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Tested-by: NetBSD Build System <jenkins@build.gluster.org>
Reviewed-by: N Balachandran <nbalacha@redhat.com>
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>