cluster/dht: Ignore ENOENT errors for unlink of linkfiles

Backport of http://review.gluster.org/4971

If unlink of linkfile returns ENOENT, do not fail unlink.
Proceed with unlinking of cached file.

Change-Id: If7cec92b40c39d68dd9c3606c6c2c3a6bd67d27b
BUG: 966848
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/6586
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>