LU-1756 kernel: cleanup lustre_compat25.h

LU-1756 kernel: cleanup lustre_compat25.h

1. SEEK_XXX is always defined
2. page_private exists since v2.6.15
3. Page_Uptodate should be replaced with PageUptodate
4. unused functions:
   ll_kernel_locked(), ll_redirty_page(), KDEVT_INIT()
   ll_path_lookup(), try_lock_page()

Signed-off-by: Peng Tao <tao.peng@emc.com>
Change-Id: I90af2e5c4678738752e0f4cade5d4302f3d1fac7
Reviewed-on: http://review.whamcloud.com/3685
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>