LU-2179 osc: truncate partial page correctly

LU-2179 osc: truncate partial page correctly

If a partial page is being truncated, the corresponding osc extent
should be held until the truncate finished.

Debug patch for osc_extent_wait() and don't wait for completion
of RPC it's not even sent in truncate.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I96a5ec1fdbb3133c735ebdfdd0330a45a2a8ab1a
Reviewed-on: http://review.whamcloud.com/4317
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>