performance/write-behind: invoke request queue processing if
we find fd marked bad while trying to fulfill lies.

* flush was queued behind some unfulfilled write.
* A previously wound write returned an error and hence fd was marked
  bad with corresponding error.
* wb_fulfill_head (invocation probably rooted in wb_flush), before
  winding checks for failures of previous writes and since there was a
  failure, calls wb_head_done without even winding one request in head.
* wb_head_done unrefs all the requests in list "head".
* since flush was last operation on fd (and most likely last operation
  on inode itself), no one invokes wb_process_queue and flush is stuck
  in request queue for eternity.

Change-Id: I3b5b114a1c401d477dd7ff64fb6119b43fda2d18
BUG: 988642
Signed-off-by: Raghavendra G <rgowdapp@redhat.com>
Reviewed-on: http://review.gluster.org/5883
Tested-by: Gluster Build System <jenkins@build.gluster.com>