distribute: Rebalance should provide even disk space distribution

Earlier disk space check had an issue which didn't
provide the needed functionality to avoid migration
when the destination had lesser available space,
scenario we need to avoid is stated below :

During rebalance `migrate-data` - Destination subvol experiences
a `reduction` in 'blocks' of free space, at the same time source
subvol gains certain 'blocks' of free space. A valid check is
necessary here to avoid errorneous move to destination where
the space could be scantily available.

This patch provides a proper fix in place by subtracting
necessary file blocks from destination and adding those blocks
to source.

Change-Id: I9c7840716a4256ef614ffc0fbfd9f2b456ac28c8
BUG: 982919
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/5961
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Shishir Gowda <sgowda@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>