cluster/distribute: Preserve file size during rebalance migration

If holes are encountered, then we do not write these to the dst,
which sometimes causes file size to be lesser than src. Data is not
corrupted, as when non-zero reads are received, we do write that data.

Calling a truncrate to give file size to prevent it from being
truncated to less than src in case the file end has holes.

Thanks to Brian Foster for providing the test case

BUG: 915554
Change-Id: I7e1e0c475118b073c3ebb87e93220c1ec22e8b7d
Signed-off-by: shishir gowda <sgowda@redhat.com>
Reviewed-on: http://review.gluster.org/4609
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
cluster/distribute: Prevent spurious multiple defrag crawls

In dht_notify, we used to create a thread to start defrag
crawls after we had heard from all child subvols.
This was in-correct, as a later event, could also trigger the
crawl again(due to the fact that all subvols had responded).

The fix is to make sure, the thread is started only once after
all subvols have responded the first time

BUG: 916449
Change-Id: I1619344fbb1cb51d5e1db38d8a29821fa870fa8b
Signed-off-by: shishir gowda <sgowda@redhat.com>
Reviewed-on: http://review.gluster.org/4610
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>