LU-9116 libcfs: avoid overflow of crypto bandwidth caculation

LU-9116 libcfs: avoid overflow of crypto bandwidth caculation

bcount and buf_len are both int, and no force convert in the caculation code:
tmp = ((bcount * buf_len / jiffies_to_msecs(end - start)) *
	1000) / (1024 * 1024);
That may cause overflow in modern fast machine.

Change-Id: I1e5abccad3e4df62907317a09de02beb6d831e13
Signed-off-by: Gu Zheng <gzheng@ddn.com>
Reviewed-on: https://review.whamcloud.com/25436
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Li Xi <lixi@ddn.com>