selftests/bpf: Use PT_REGS_SYSCALL_REGS in bpf_syscall_macro

Ensure that PT_REGS_SYSCALL_REGS works correctly.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220209021745.2215452-4-iii@linux.ibm.com

tools/testing/selftests/bpf/progs/bpf_syscall_macro.c

@@ -33,7 +33,7 @@ int BPF_KPROBE(handle_sys_prctl)
 	if (pid != filter_pid)
 		return 0;
 
-	real_regs = (struct pt_regs *)PT_REGS_PARM1(ctx);
+	real_regs = PT_REGS_SYSCALL_REGS(ctx);
 
 	/* test for PT_REGS_PARM */