nfs: 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().

Change-Id: I2bf68410dd86cdf9cfe8a3d43adc27497d8bb36f
BUG: 959190
Signed-off-by: Rajesh Amaravathi <rajesh@redhat.com>
Reviewed-on: http://review.gluster.org/4943
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Amar Tumballi <amarts@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>