cluster/dht: If linkfile unlink fails with ENOTCONN, do not fail

Currently if linkfile fails with ENOENT, we do not fail. We also
need to treat failures with ENOTCONN as success, as if cached subvol
is up, rm of a file should succeed. A stale linkfile will get removed
later

Change-Id: I71d136847933351ed9e2c939bda4a69bc96a3cfc
BUG: 983416
Signed-off-by: shishir gowda <sgowda@redhat.com>
Reviewed-on: http://review.gluster.org/5317
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>