staging/lustre/llite: allocate and free client cache asynchronously

Since the inflight request holds import refcount as well as export,
sometimes obd_disconnect() in client_common_put_super() can't put
the last refcount of OSC import (e.g. due to network disconnection),
this will cause cl_cache being accessed after free.

To fix this issue, ccc_users is used as cl_cache refcount, and
lov/llite/osc all hold one cl_cache refcount respectively, to avoid
the race that a new OST is being added into the system when the client
is mounted.
The following cl_cache functions are added:
- cl_cache_init(): allocate and initialize cl_cache
- cl_cache_incref(): increase cl_cache refcount
- cl_cache_decref(): decrease cl_cache refcount and free the cache
  if refcount=0.

Signed-off-by: Emoly Liu <[email protected]>
Reviewed-on: http://review.whamcloud.com/13746
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-6173
Reviewed-by: Niu Yawei <[email protected]>
Signed-off-by: Oleg Drokin <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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