LU-3274 osc: allow to call brw_commit() multiple times

LU-3274 osc: allow to call brw_commit() multiple times

Sometimes the rq_commit_cb of BRW RPC can be called twice if that RPC
has already committed at reply time. This will cause inaccuracy of
unstable pages accounting and then assertion.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: Ic596e1d44e2c27bf13701d2c6c9691562beb7e58
Reviewed-on: http://review.whamcloud.com/8215
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Prakash Surya <surya1@llnl.gov>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>