staging: lustre: statahead: statahead thread wait for RPCs to finish

Statahead thread should wait for inflight stat RPCs to finish in
case statahead RPC callback may access data allocated in statahead
thread context.

ll_sa_entry_fini() should keep old entry if stat RPC is not
finished yet.

Simplify sai refcounting:
* newly allocated sai will hold one refcount, and it will put it
  after starting statahead thread.
* statahead thread holds one refcount.
* agl thread holds one refcount.
* stat process calls do_statahead_enter() which will try to get
  sai, and if it's valid, it will revalidate from statahead cache,
  and put refcount after use.

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3270
Reviewed-on: http://review.whamcloud.com/9663


Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>