selftests/bpf: Fix test_lsm on s390x

Use syscall macros to access the setdomainname() arguments; currently
the code uses gprs[2] instead of orig_gpr2 for the first argument.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-16-iii@linux.ibm.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -6,6 +6,7 @@
 
 #include "bpf_misc.h"
 #include "vmlinux.h"
+#include <bpf/bpf_core_read.h>
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_tracing.h>
 #include <errno.h>
@@ -164,8 +165,8 @@ int copy_test = 0;
 SEC("fentry.s/" SYS_PREFIX "sys_setdomainname")
 int BPF_PROG(test_sys_setdomainname, struct pt_regs *regs)
 {
-	void *ptr = (void *)PT_REGS_PARM1(regs);
-	int len = PT_REGS_PARM2(regs);
+	void *ptr = (void *)PT_REGS_PARM1_SYSCALL(regs);
+	int len = PT_REGS_PARM2_SYSCALL(regs);
 	int buf = 0;
 	long ret;