dma-mapping: benchmark: pretend DMA is transmitting

mainline inclusion
commit id 9dc00b25
from mainline-v5.12-rc1-dontuse
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I60AX5
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9dc00b25eadf2908ae76ac0607b55a9f4e0e0cdc



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In a real dma mapping user case, after dma_map is done, data will be
transmit. Thus, in multi-threaded user scenario, IOMMU contention
should not be that severe. For example, if users enable multiple
threads to send network packets through 1G/10G/100Gbps NIC, usually
the steps will be: map -> transmission -> unmap.  Transmission delay
reduces the contention of IOMMU.

Here a delay is added to simulate the transmission between map and unmap
so that the tested result could be more accurate for TX and simple RX.
A typical TX transmission for NIC would be like: map -> TX -> unmap
since the socket buffers come from OS. Simple RX model eg. disk driver,
is also map -> RX -> unmap, but real RX model in a NIC could be more
complicated considering packets can come spontaneously and many drivers
are using pre-mapped buffers pool. This is in the TBD list.

Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jie Liu <liujie375@h-partners.com>