LU-12624 obdclass: lu_tgt_descs cleanup

LU-12624 obdclass: lu_tgt_descs cleanup

This patch cleans up code about lu_tgt_descs, so that it's cleaner
to add MDT object QoS allocation support:
* rename struct ost_pool to lu_tgt_pool.
* put struct lu_qos, lmv_desc/lov_desc and lu_tgt_pool into struct
  lu_tgt_descs because it's more natural to manage these data there
  and fewer arguments are needed to pass around in related functions.
* remove lu_tgt_descs.ltd_tgtnr, use
  lu_tgt_descs.ltd_lov_desc.ld_tgt_count instead, because they are
  duplicate.
* other cleanups.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I46f2e0ff06a8e580bac1dfda9a09a549b38d487d
Reviewed-on: https://review.whamcloud.com/35824
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>