Add return value check and handling for xattr loading

libbpf_find_vmlinux_btf_id may return error code for different reasons. We
should check and report the error we have just got, rather than let it slip
down the path and give an ambiguous error later.

Signed-off-by: He Zhe <zhe.he@windriver.com>

src/cc/libbpf.c

@@ -588,8 +588,18 @@ int bcc_prog_load_xattr(struct bpf_load_program_attr *attr, int prog_len,
 
     if (attr->prog_type == BPF_PROG_TYPE_TRACING ||
         attr->prog_type == BPF_PROG_TYPE_LSM) {
-      attr->attach_btf_id = libbpf_find_vmlinux_btf_id(attr->name + name_offset,
-                                                       expected_attach_type);
+      ret = libbpf_find_vmlinux_btf_id(attr->name + name_offset,
+                                       expected_attach_type);
+      if (ret == -EINVAL) {
+        fprintf(stderr, "bpf: vmlinux BTF is not found\n");
+        return ret;
+      } else if (ret < 0) {
+        fprintf(stderr, "bpf: %s is not found in vmlinux BTF\n",
+                attr->name + name_offset);
+        return ret;
+      }
+
+      attr->attach_btf_id = ret;
       attr->expected_attach_type = expected_attach_type;
     }