LU-1303 lod: improvements and fixes

LU-1303 lod: improvements and fixes

- osp_statfs() returns -ENOTCONN if the corresponded OST found
  not connected. this let us to remove few additional checks in
  the allocation policy functions.

- struct obd_statfs gets new field: os_fprecreated
  LOD uses this to skip OSPs with no objects ready to use

- osp_statfs() returns number of already precerated objects
  in new os_fprecreated field

- OS_STATE_DEGRADED is ignored on the first 2 passes in RR policy

- lod_alloc_specific() to verify and skip OSPs already used in
  striping

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Change-Id: I86351bc1dcca7182bc5adf4eb3e03c054e33e95f
Reviewed-on: http://review.whamcloud.com/4242
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Mike Pershin <tappro@whamcloud.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>