performance/write-behind: Enable write-behind when strict_O_DIRECT is not set.

When open() with O_DIRECT happens, write-behind was being disabled for the
fd irrespective of strict_O_DIRECT option. This commit disables write-behind
only when strict_O_DIRECT is enabled.

Change-Id: Ieef180e52910c3bf64d46b26b0e5dc3b8542f6d2
BUG: 923556
Signed-off-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-on: http://review.gluster.org/4697
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-on: http://review.gluster.org/5108
Reviewed-by: Anand Avati <avati@redhat.com>