tracing/kprobes, x86/ptrace: Fix regs argument order for i386

On i386, the order of parameters passed on regs is eax,edx,and ecx
(as per regparm(3) calling conventions).

Change the mapping in regs_get_kernel_argument(), so that arg1=ax
arg2=dx, and arg3=cx.

Running the selftests testcase kprobes_args_use.tc shows the result
as passed.

Fixes: 3c88ee1 ("x86: ptrace: Add function argument access API")
Signed-off-by: Vamshi K Sthambamkadi <vamshi.k.sthambamkadi@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/20200828113242.GA1424@cosmos

arch/x86/include/asm/ptrace.h

@@ -327,8 +327,8 @@ static inline unsigned long regs_get_kernel_argument(struct pt_regs *regs,
 	static const unsigned int argument_offs[] = {
 #ifdef __i386__
 		offsetof(struct pt_regs, ax),
-		offsetof(struct pt_regs, cx),
 		offsetof(struct pt_regs, dx),
+		offsetof(struct pt_regs, cx),
 #define NR_REG_ARGUMENTS 3
 #else
 		offsetof(struct pt_regs, di),