LU-4540 llite: deadlock for page write

LU-4540 llite: deadlock for page write

Writing thread already locked page #1, and then wait for the
Writeback bit of page #2;

Ptlrpc thread is composing a write RPC, so it sets Writeback on
page #2 and tries to lock page #1 to make it ready.

Deadlocked.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I2da547b4c93c3464e520a1f593985adae9360bc9
Reviewed-on: http://review.whamcloud.com/9036
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>