LU-56 lnet: code cleanup for lib-md.c

LU-56 lnet: code cleanup for lib-md.c

This patch just did some code cleanup for lib-md.c, it's an
intermediate patch for LNet SMP improvements.

Signed-off-by: Liang Zhen <liang@whamcloud.com>
Change-Id: I5d162852008284ae08f3b9e7935c9251b245fb5c
Reviewed-on: http://review.whamcloud.com/3010
Tested-by: Hudson
Reviewed-by: Doug Oucharek <doug@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Bobi Jam <bobijam@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>