samples/bpf: Fix test_map_in_map on s390

s390 uses socketcall multiplexer instead of individual socket syscalls.
Therefore, "kprobe/" SYSCALL(sys_connect) does not trigger and
test_map_in_map fails. Fix by using "kprobe/__sys_connect" instead.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200915115519.3769807-1-iii@linux.ibm.com

samples/bpf/test_map_in_map_kern.c

@@ -103,19 +103,18 @@ static __always_inline int do_inline_hash_lookup(void *inner_map, u32 port)
 	return result ? *result : -ENOENT;
 }
 
-SEC("kprobe/" SYSCALL(sys_connect))
+SEC("kprobe/__sys_connect")
 int trace_sys_connect(struct pt_regs *ctx)
 {
-	struct pt_regs *real_regs = (struct pt_regs *)PT_REGS_PARM1_CORE(ctx);
 	struct sockaddr_in6 *in6;
 	u16 test_case, port, dst6[8];
 	int addrlen, ret, inline_ret, ret_key = 0;
 	u32 port_key;
 	void *outer_map, *inner_map;
 	bool inline_hash = false;
 
-	in6 = (struct sockaddr_in6 *)PT_REGS_PARM2_CORE(real_regs);
-	addrlen = (int)PT_REGS_PARM3_CORE(real_regs);
+	in6 = (struct sockaddr_in6 *)PT_REGS_PARM2_CORE(ctx);
+	addrlen = (int)PT_REGS_PARM3_CORE(ctx);
 
 	if (addrlen != sizeof(*in6))
 		return 0;