LU-506 kernel: 2.6.38 kernel macro check cleanup

* HAVE_DCACHE_LOCK is used to mark kernel >= 2.6.38, instead of other
kernel macros, this makes it easier to maintain kernel support in
the future.
* minor cleanups.

Signed-off-by: Lai Siyao <laisiyao@whamcloud.com>
Change-Id: Ic9a6a8c339a10b34e2b1b06b47fd9e252cd3ec2c
Reviewed-on: http://review.whamcloud.com/3159
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>