selftests/bpf: Add test to access int ptr argument in tracing program

Adding verifier test for accessing int pointer argument in
tracing programs.

The test program loads 2nd argument of bpf_modify_return_test
function which is int pointer and checks that verifier allows
that.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211208193245.172141-3-jolsa@kernel.org

tools/testing/selftests/bpf/verifier/btf_ctx_access.c

+{
+	"btf_ctx_access accept",
+	.insns = {
+	BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1, 8),	/* load 2nd argument value (int pointer) */
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.result = ACCEPT,
+	.prog_type = BPF_PROG_TYPE_TRACING,
+	.expected_attach_type = BPF_TRACE_FENTRY,
+	.kfunc = "bpf_modify_return_test",
+},