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: If3ae0bf5a44f8739ce35b3ee3f191009ddd44455
BUG: 858488
Signed-off-by: Raghavendra Talur <rtalur@redhat.com>
Reviewed-on: http://review.gluster.org/4420
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>