LU-10448 lod: pick primary mirror for write

LU-10448 lod: pick primary mirror for write

As a mirrored file being written for the first time, MDS will choose
a mirror to write the data, a primary choosing policy function is
defined in this patch (lod_primary_pick()) to avoid the mirror with
unavailable OSTs.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: I5d6d0459e96583294c3040a7994c33114be1e439
Reviewed-on: https://review.whamcloud.com/30711
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>