LU-506 FC15: unplug_fn removed since 2.6.39.

LU-506 FC15: unplug_fn removed since 2.6.39.

The old plugging interface isn't exist any more. So
disable the unplug_fn callback in this case.

Signed-off-by: Yang Sheng <ys@whamcloud.com>
Change-Id: I2396b845e2f2ef158f8e21e031a31ac83f3a5b02
Reviewed-on: http://review.whamcloud.com/1738
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <laisiyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>