Description of problem:
=======================
Many files were not migrated from the decommissioned bricks; commit results in data loss.

Version-Release number of selected component (if applicable):
3.12.2-5.el7rhgs.x86_64

How reproducible:
Reporting at first occurrence

Steps to Reproduce:
===================
1) Create a x3 volume with brick-mux enabled and start it.
2) FUSE mount it on multiple clients.
3) From Client-1 : run script to create folders and files continuously 
 From client-2 : start linux kernel untar
 From client-3 : while true;do find;done
 From client-4 : while true;do ls -lRt;done
4) While step-3 is in-progress, killed server-1 brick process using kill -9 <pid>. 
As brick mux is enabled killing single brick on the server using kill -9 would take down all the bricks on the node.
5) Now, add 3 bricks to the volume and after few secs immediately start removing old bricks.
6) Wait for remove-brick to complete.

Actual results:
===============
Many files were not migrated from the decommissioned bricks; commit results in data loss.

Expected results:
=================
Remove-brick operation should migrate all the files from the decommissioned brick.