LU-1756 kernel: cleanup lustre_compat25.h

LU-1756 kernel: cleanup lustre_compat25.h

1. unused functions:
   ll_remove_suid(), labs()
2. abs() is always defined
3. use SLAB_DESTROY_BY_RCU directly
4. call sb_any_quota_loaded() directl

Signed-off-by: Peng Tao <tao.peng@emc.com>
Change-Id: Ic52e1a75f3d71887ae91f03a465112ab6c08c746
Reviewed-on: http://review.whamcloud.com/3688
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>