csky: Fix syscall_get_arguments() and syscall_set_arguments()

C-SKY syscall arguments are located in orig_a0,a1,a2,a3,regs[0],regs[1]
fields of struct pt_regs.

Due to an off-by-one bug and a bug in pointer arithmetic
syscall_get_arguments() was reading orig_a0,regs[1..5] fields instead.
Likewise, syscall_set_arguments() was writing orig_a0,regs[1..5] fields
instead.

Link: http://lkml.kernel.org/r/[e-mail]

Fixes: 4859bfca11c7d ("csky: System Call")
Cc: Ingo Molnar <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Will Drewry <[e-mail]>
Cc: [e-mail] # v4.20+
Tested-by: Guo Ren <[e-mail]>
Acked-by: Guo Ren <[e-mail]>
Signed-off-by: Dmitry V. Levin <[e-mail]>
Signed-off-by: Steven Rostedt (VMware) <[e-mail]>

arch/csky/include/asm/syscall.h

···
 	if (i == 0) {
 		args[0] = regs->orig_a0;
 		args++;
-		i++;
 		n--;
+	} else {
+		i--;
 	}
-	memcpy(args, &regs->a1 + i * sizeof(regs->a1), n * sizeof(args[0]));
+	memcpy(args, &regs->a1 + i, n * sizeof(args[0]));
 }
 
 static inline void
···
 	if (i == 0) {
 		regs->orig_a0 = args[0];
 		args++;
-		i++;
 		n--;
+	} else {
+		i--;
 	}
-	memcpy(&regs->a1 + i * sizeof(regs->a1), args, n * sizeof(regs->a0));
+	memcpy(&regs->a1 + i, args, n * sizeof(regs->a1));
 }
 
 static inline int