LU-1373 ptlrpc: add flow control extension to ptlrpc req set

LU-1373 ptlrpc: add flow control extension to ptlrpc req set

This patch allows new requests to be added to a request set while
this latter has already requests in flight. This is done by adding a
callback function invoked by the request set to generate RPCs.
The request set will fire a new RPC each time one completes, keeping
the number of RPCs in flight equals to set->set_max_inflight.

Lock callbacks can thus be sent by the service thread again. This
avoids doing disk I/O from the ptlrpcd context.

Signed-off-by: Johann Lombardi <johann@whamcloud.com>
Change-Id: If95922fa8da1dfa7ce7c98d6bfe35e9a5f5bb34f
Reviewed-on: http://review.whamcloud.com/2650
Tested-by: Hudson
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>