performance/open-behind: use anonymous fd for doing fstat and readv

Change-Id: I61a3c221e0a15736ab6315e2538c03dac27480a5
BUG: 846240
Signed-off-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-on: http://review.gluster.org/4483
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>