LU-1028 osc: fix grant checking on the osc side

If there is no available grant on the client side, but exists dirty
pages, we should issue BRW rpc and wait for more grants.

Another changes is about lost_grant: if dirty pages were truncated,
grants will be returned to client, instead of the target having
allocated them.

Signed-off-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Change-Id: I50791554807dddfafafcd2b8d228958ebfa3ac16
Reviewed-on: http://review.whamcloud.com/2037
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: wangdi <di.wang@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>