cluster/distribute: Rebalance should also verify free inodes

Currently during `MIGRATE_DATA` we never verified about the total
inode usage among new and old bricks. Such checks are available for
disk space usage but it is also needed for inodes during data
migration. Such a check leads to uniform outcome of file distribution
upon rebalance.

Patch provides:

- Check dst_inodes < src_inodes, a friendly `warning` message to
  indicate we have to ignore such an attempt
- Rename __dht_check_free_space() --> __dht_check_free_space_and_inodes()

Change-Id: I7bc4dd8b507883f0fb836300e99f0bb083493f5f
BUG: 982919
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/5948
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>