LU-11025 obdclass: add lu_device_operations::ldo_fid_alloc()

LU-11025 obdclass: add lu_device_operations::ldo_fid_alloc()

Add an interface ldo_fid_alloc in lu_device_operations, which is to
allocate a FID by parent object and sub file name, this will be
used to migrate sub files for directory restripe.

The existing osd_fid_alloc() and osp_fid_alloc() will switch to this
interface from obd_ops::o_fid_alloc().

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Ide480fe492fd2d4d5de675bbc61aee7e2a9e3ce3
Reviewed-on: https://review.whamcloud.com/37282
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>