LU-5717 ptlrpc: fix deadlock problem of nrs_tbf_timer_cb

LU-5717 ptlrpc: fix deadlock problem of nrs_tbf_timer_cb

When callback of TBF timer is triggered, nrs_lock could be
held by the current CPU which will cause dead lock. This
patch removes unnecessary nrs_lock to fix this problem.

Signed-off-by: Li Xi <lixi@ddn.com>
Change-Id: I6329e3e71da30a415dbb35b37d79ade118917c6a
Reviewed-on: http://review.whamcloud.com/12228
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>