MAINTAINERS: update maintainer list of DMA MAPPING BENCHMARK

Barry Song will not focus on this area, and Xiang Chen will continue his
work to maintain this module.

Signed-off-by: Xiang Chen <chenxiang66@hisilicon.com>
Acked-by: Barry Song <song.bao.hua@hisilicon.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

Author: Xiang Chen

MAINTAINERS

@@ -5765,7 +5765,7 @@ F:	include/linux/dma-map-ops.h
 F:	kernel/dma/
 
 DMA MAPPING BENCHMARK
-M:	Barry Song <song.bao.hua@hisilicon.com>
+M:	Xiang Chen <chenxiang66@hisilicon.com>
 L:	iommu@lists.linux-foundation.org
 F:	kernel/dma/map_benchmark.c
 F:	tools/testing/selftests/dma/