staging/lustre/llite: access layout version under a lock

We used to access layout version under the protection of ldlm
lock, this introduces extra overhead for dlm lock matching.

In this patch, lli_layout_lock is introduced to access the layout
version. Also, when a layout lock is losing, we should tear down
mmap of the correspoding inode to avoid stale data accessing in the
future.

This is part of technical verification of replication.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-on: http://review.whamcloud.com/8689
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3254


Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>