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.