LU-1756 kernel: cleanup lustre_compat25.h

LU-1756 kernel: cleanup lustre_compat25.h

1. unused functions:
   ll_bdi_wb_cnt()
2. call bdi_init/bdi_destroy directly
3. call add_to_page_cache_lru directly
4. fix a typo of ll_pagevec_init

Signed-off-by: Peng Tao <tao.peng@emc.com>
Change-Id: I0ce799bc5aad70279a652cf911b9880abcc74194
Reviewed-on: http://review.whamcloud.com/4656
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Yang Sheng <yang.sheng@intel.com>