LU-4684 mdt: improve directory stripe lock

LU-4684 mdt: improve directory stripe lock

Striped directory has an implication that the first stripe is
local, and others are remote, but this is not true for migrating
directory because its stripes consists of both the original and
the newly created stripes.

This patch also put striped directory master object locking and
stripes locking into one function called mdt_reint_striped_lock(),
which simplifies locking code.

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: I4724447e5b10c301b6799e1827f6d13a40876945
Reviewed-on: https://review.whamcloud.com/31425
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>