LU-6174 nrs: perform proper division

LU-6174 nrs: perform proper division

For NRS TBF both tr_nsecs and tr_rpc_rate are 64 bit values. The
code is wrongly using do_div() which could truncate tr_rpc_rate
to a 32 bit value. With current hardware setups the rpc rate is
less than the 4 billion limit of u32, currently I see 64K of rpcs
with a single IB port. Since this is the case we can change
tr_rpc_rate to a 32 bit value.

Change-Id: I62eabf3a8a30f5c9062aaf4c32fa878710a788c8
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/36749
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>