LU-5531 osc: Debug to match extent to brw RPC

LU-5531 osc: Debug to match extent to brw RPC

Currently, it's difficult to match brw RPCs to objects and
extents from client logs.  This patch adds a D_RPCTRACE
debug message giving the necessary information.

Signed-off-by: Patrick Farrell <paf@cray.com>
Change-Id: Ic3470a805f00b5d9c0f6f72470d3f3eb48a10b7a
Reviewed-on: http://review.whamcloud.com/11548
Tested-by: Jenkins
Reviewed-by: Alexey Lyashkov <alexey.lyashkov@seagate.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Ann Koehler <amk@cray.com>
Reviewed-by: Ryan Haasken <haasken@cray.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>