staging: lustre: obdclass: unified flow control interfaces

Unify the flow control interfaces for MDC RPC and FLD RPC.
We allow to adjust the maximum inflight RPCs count via /sys
interface.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4687
Reviewed-on: http://review.whamcloud.com/9562
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.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>