gNFS: avoid double fd unref in opendir

Noticed that the fd_unref was called on the fd regardless of the return
value at nfs3svc_opendir_readdir_cbk(), hence removing an extra unref in
the negative case in nfs_inode_opendir_cbk, which fixes the spurious
fd_unref().

Back port of: http://review.gluster.org/4943 (Rajesh Amaravathi)

Change-Id: Ibddf487c7890407d01befedd65eefb10cb9c989f
BUG: 1011761
Signed-off-by: Santosh Kumar Pradhan <spradhan@redhat.com>
Reviewed-on: http://review.gluster.org/5996
Reviewed-by: Rajesh Joseph <rjoseph@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>