cluster/dht: getxattr linkto as root:root

In path based op's like truncate, we use getxattr instead of
fgetxattr call. These can fail with permission denied issues
as linkto file creation, and setattr of ownership is not atomic,
and in cases where setattr failed (subvols down..)

The fix is to perform getxattr as root:root as it is a internal
fop. fgetxattr, bypass the access check, as it already has a valid
open fd.

Change-Id: Ie221c9172e3c1c7ed4e50c8782d362826910756f
BUG: 957074
Signed-off-by: shishir gowda <sgowda@redhat.com>
Reviewed-on: http://review.gluster.org/4890
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>