dma-mapping: benchmark: Restore padding to ensure uABI remained consistent

The padding field in the structure was previously reserved to
maintain a stable interface for potential new fields, ensuring
compatibility with user-space shared data structures.
However,it was accidentally removed by tiantao in a prior commit,
which may lead to incompatibility between user space and the kernel.

This patch reinstates the padding to restore the original structure
layout and preserve compatibility.

Fixes: 8ddde07a3d28 ("dma-mapping: benchmark: extract a common header file for map_benchmark definition")
Cc: [e-mail]
Acked-by: Barry Song <[e-mail]>
Signed-off-by: Qinxin Xia <[e-mail]>
Reported-by: Barry Song <[e-mail]>
Closes: https://lore.kernel.org/lkml/[e-mail]/
Reviewed-by: Jonathan Cameron <[e-mail]>
Signed-off-by: Marek Szyprowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

include/linux/map_benchmark.h

···
 	__u32 dma_dir; /* DMA data direction */
 	__u32 dma_trans_ns; /* time for DMA transmission in ns */
 	__u32 granule;  /* how many PAGE_SIZE will do map/unmap once a time */
+	__u8 expansion[76]; /* For future use */
 };
 #endif /* _KERNEL_DMA_BENCHMARK_H */