libglusterfs/syncop: do not hold ref on the fd in cbk

* Do not do fd_ref in cbks of the fops which return a fd (such as
  open, opendir, create).

Change-Id: Ic2f5b234c5c09c258494f4fb5d600a64813823ad
BUG: 885008
Signed-off-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-on: http://review.gluster.org/4282
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>