LU-11753 obdclass: index_page support variable length rec

LU-11753 obdclass: index_page support variable length rec

mdd_dir_is_empty() may readdir from other MDT if directory
is striped or remote, in this case, it will issue OBD_IDX_READ
RPC to fetch dir page, and on remote MDT dt_index_page_build()
is called to build page, but this function doesn't support
variable length record, so it may miscalculate offset in
reading, which may cause crash.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: Ia25a0aca52fb1323ea64a7ff72bf6022754af32c
Reviewed-on: https://review.whamcloud.com/33837
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Stephan Thiell <sthiell@stanford.edu>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>