LU-709 compat: remove all reference to LINUX_VERSION_CODE

LU-709 compat: remove all reference to LINUX_VERSION_CODE

All LINUX_VERSION_CODE refereces are used to note kernel version older
than 2.6.18. So we can drop them all.

Affected variables are inlined as much as possible to minimize compat
code definition.

Signed-off-by: Peng Tao <tao.peng@emc.com>
Change-Id: If3b94ceb5e21db8f96b11526399e9cd8f174fd6f
Reviewed-on: http://review.whamcloud.com/3442
Tested-by: Hudson
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Prakash Surya <surya1@llnl.gov>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>