Handle layout change internally when mapping name to stripe:
* Move layout changing related code into lmv name to stripe mapping
  so callers doesn't need to take care of the internals.
* lmv_name_to_stripe_index() maps name in new layout, and
  lmv_name_to_stripe_index_old() in old layout.
* rename lmv_migrate_existence_check() to lmv_old_layout_lookup()
  to support directory restripe in the future.
* support layout changing directory in LFSCK.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Icf8bda5db884784f761a2d373a6f81d7e13f525f
Reviewed-on: https://review.whamcloud.com/37711


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>