Description of problem:
=======================
When we add a brick which is of smaller size, when we trigger rebalance, files are not moved from the src_brick to dst_brick if src_available_space >dst_available_space
Below are the problems due to this situation
1)we must not be checking dst_available_space but , rather see if the file being migrated has sufficient space to fit in the dst_brick or not
2)even though we are not migrating, we scan whole filesystem, everytime a rebalance is triggered. This is waste of cpu cycles. 
Agree, that layout fixing happens, but fixing would have happened as part of first rebalance completion. We don't have to probably try to fix everytime, given that when i am writing new files, they are anyways getting distributed b/w bricks, and it is only old files, which needs to be rebalanced(ie due to add brick), and first rebalance would have ideally done fix layout. yes there can be exceptions, like the layout fixing may not have been compeleted due to abort or stop by user or unexpected events



Version-Release number of selected component (if applicable):
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3.8.4-35


How reproducible:
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always

Steps to Reproduce:
1.create a distribute(or distrep) volume with all bricks of say 10GB
2.do some IOs
3.now add brick of size lesser than 10gb say 5gb
3.trigger rebalance
4. add new files
5. from backend, add a file of 5gb on old bricks(to make dst_avail_size >src_avail_size)

Actual results:
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after step 3, rebalance completes, but no files are migrated
with step4, new files are distributed as per hashing
with step 5, now rebalance will migrate some old files to dst, but it does only till src_avail_size <dst_avail_size