selftests dma: fix compile error for dma_map_benchmark

When building selftests/dma:
$ make -C tools/testing/selftests TARGETS=dma
I hit the following compilation error:

dma_map_benchmark.c:13:10: fatal error: linux/map_benchmark.h: No such file or directory
 #include <linux/map_benchmark.h>
          ^~~~~~~~~~~~~~~~~~~~~~~

dma/Makefile does not include the map_benchmark.h path, so add
more including path, and fix include order in dma_map_benchmark.c

Fixes: 8ddde07a ("dma-mapping: benchmark: extract a common header file for map_benchmark definition")
Signed-off-by: Yu Liao <liaoyu15@huawei.com>
Tested-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>