bpf, selftests: Add tests for sock_ops load with r9, r8.r7 registers

Loads in sock_ops case when using high registers requires extra logic to
ensure the correct temporary value is used. We need to ensure the temp
register does not use either the src_reg or dst_reg. Lets add an asm
test to force the logic is triggered.

The xlated code is here,

  30: (7b) *(u64 *)(r9 +32) = r7
  31: (61) r7 = *(u32 *)(r9 +28)
  32: (15) if r7 == 0x0 goto pc+2
  33: (79) r7 = *(u64 *)(r9 +0)
  34: (63) *(u32 *)(r7 +916) = r8
  35: (79) r7 = *(u64 *)(r9 +32)

Notice r9 and r8 are not used for temp registers and r7 is chosen.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/159718353345.4728.8805043614257933227.stgit@john-Precision-5820-Tower

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

@@ -75,6 +75,13 @@ int bpf_testcb(struct bpf_sock_ops *skops)
 		: [skops] "r"(skops)
 		:);
 
+	asm volatile (
+		"r9 = %[skops];\n"
+		"r8 = *(u32 *)(r9 +164);\n"
+		"*(u32 *)(r9 +164) = r8;\n"
+		:: [skops] "r"(skops)
+		: "r9", "r8");
+
 	op = (int) skops->op;
 
 	update_event_map(op);