cluster/dht: Rename should not fail post hardlink creation

In the rename path, we wind the creation of newname hardlink and
linkto file in dst hashed a the same time. If the linkto creation
fails, but the link creation succeeds, we enter the failure code
and cleanup the created newname hardlink.

In the interim if another client looks up newname and finds it as
a hardlink from FUSE, it could send an unlink for oldname instead
of a rename. This combined with the above cleanup code could end
up losing all the files copies, and thereby losing data.

This fix separates these steps into 2 parts, creating the linkto
first and then the link file, so that post link file creation no
failures would cleanup the newname file. If linkto fails then link
is not attempted, thereby not polluting the name space with
newname.

Change-Id: I61da8e906060da16a31ea1076eec2f01fd617f44
BUG: 1130888
Signed-off-by: Shyam <srangana@redhat.com>
Reviewed-on: http://review.gluster.org/8570
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>