LU-709 build: clean up percpu_counter and sb.s_time_gran

LU-709 build: clean up percpu_counter and sb.s_time_gran

1. linux/percpu_counter.h exists since v2.6.12,
   percpu_counter_init() takes two arguments since v2.6.17,
   and percpu_counter is not used in lustre
2. superblock.s_time_gran exists since v2.6.12
3. get_sb_time_gran() doesn't exist since v2.6.12

Signed-off-by: Peng Tao <tao.peng@emc.com>
Change-Id: Id80b0bfc702f509218fcf29d01334894d122b9c8
Reviewed-on: http://review.whamcloud.com/2317
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>