cluster/dht: Correct min_free_disk behaviour

Problem:
Files were being created in subvol which had less than min_free_disk available
even in the cases where other subvols with more space were available.

Solution:
Changed the logic to look for subvol which has more space available. In cases
where all the subvols have lesser than Min_free_disk available , the one with
max space and atleast one inode is available.

Known Issue: Cannot ensure that first file that is created right after
min-free-value is crossed on a brick will get created in other brick because
disk usage stat takes some time to update in glusterprocess. Will fix that as
part of another bug.

Change-Id: Icaba552db053ad8b00be0914b1f4853fb7661bd3
BUG: 874554
Signed-off-by: Raghavendra Talur <rtalur@redhat.com>
Signed-off-by: Varun Shastry <vshastry@redhat.com>
Reviewed-on: http://review.gluster.org/4839
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>