LU-15926 nrs: fix tbf realtime rules

LU-15926 nrs: fix tbf realtime rules

tc_nsecs_resid should be reset to 0 when changing a rule otherwise
this could lead to mds crashes for realtime policies.

nrs_tbf_req_get(): ASSERTION( cli->tc_nsecs_resid < cli->tc_nsecs )

Fixes: d11fa2c27959 ("LU-9228 nrs: TBF realtime policies under congestion")
Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Change-Id: I280acb42e104088c6b8750a0bb7bf9c50cf96e73
Reviewed-on: https://review.whamcloud.com/47585
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>