dht: make linkfile creation mode explicitly get set

Because of posix default_acl on parent directory, the mode
of linkfile can get masked with the mode in the default acl.

This breaks DHT integrity. So let the mode get explicitly reset
after mknod().

Change-Id: Ia7328e1ee7b4430bda308f9da293dba78405e081
BUG: 990410
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5440
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Raghavendra Talur <rtalur@redhat.com>