dma-debug: only skip one stackframe entry

With skip set to 1, I get a traceback like this:

[  106.867637] DMA-API: Mapped at:
[  106.870784]  afu_dma_map_region+0x2cd/0x4f0 [dfl_afu]
[  106.875839]  afu_ioctl+0x258/0x380 [dfl_afu]
[  106.880108]  do_vfs_ioctl+0xa9/0x720
[  106.883688]  ksys_ioctl+0x60/0x90
[  106.887007]  __x64_sys_ioctl+0x16/0x20

With the previous value of 2, afu_dma_map_region was being omitted.  I
suspect that the code paths have simply changed since the value of 2 was
chosen a decade ago, but it's also possible that it varies based on which
mapping function was used, compiler inlining choices, etc.  In any case,
it's best to err on the side of skipping less.
Signed-off-by: NScott Wood <swood@redhat.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>

kernel/dma/debug.c

@@ -706,7 +706,7 @@ static struct dma_debug_entry *dma_entry_alloc(void)
 #ifdef CONFIG_STACKTRACE
 	entry->stacktrace.max_entries = DMA_DEBUG_STACKTRACE_ENTRIES;
 	entry->stacktrace.entries = entry->st_entries;
-	entry->stacktrace.skip = 2;
+	entry->stacktrace.skip = 1;
 	save_stack_trace(&entry->stacktrace);
 #endif