selftests/bpf: improve error messages in veristat

Return error if preset parsing fails. Avoid proceeding with veristat run
if preset does not parse.
Before:
```
./veristat set_global_vars.bpf.o -G "arr[999999999999999999999] = 1"
Failed to parse value '999999999999999999999'
Processing 'set_global_vars.bpf.o'...
File Program Verdict Duration (us) Insns States Program size Jited size
--------------------- ---------------- ------- ------------- ----- ------ ------------ ----------
set_global_vars.bpf.o test_set_globals success 27 64 0 82 0
--------------------- ---------------- ------- ------------- ----- ------ ------------ ----------
Done. Processed 1 files, 0 programs. Skipped 1 files, 0 programs.
```
After:
```
./veristat set_global_vars.bpf.o -G "arr[999999999999999999999] = 1"
Failed to parse value '999999999999999999999'
Failed to parse global variable presets: arr[999999999999999999999] = 1
```

Improve error messages:
* If preset struct member can't be found.
* Array index out of bounds

Extract rtrim function.

Signed-off-by: Mykyta Yatsenko <[e-mail]>
Acked-by: Eduard Zingerman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/veristat.c

···
 	return c == 'v' || c == 'V' || c == '.' || c == '!' || c == '_';
 }
 
+static char *rtrim(char *str)
+{
+	int i;
+
+	for (i = strlen(str) - 1; i > 0; --i) {
+		if (!isspace(str[i]))
+			break;
+		str[i] = '\0';
+	}
+	return str;
+}
+
 static int parse_stat(const char *stat_name, struct stat_specs *specs)
 {
 	int id;
···
 static int parse_var_atoms(const char *full_var, struct var_preset *preset)
 {
 	char expr[256], var[256], *name, *saveptr;
-	int n, len, off;
+	int n, len, off, err;
 
 	snprintf(expr, sizeof(expr), "%s", full_var);
 	preset->atom_count = 0;
···
 			fprintf(stderr, "Can't parse %s\n", name);
 			return -EINVAL;
 		}
-		append_preset_atom(preset, var, false);
+		err = append_preset_atom(preset, var, false);
+		if (err)
+			return err;
 
 		/* parse optional array indexes */
 		while (off < len) {
···
 				fprintf(stderr, "Can't parse %s as index\n", name + off);
 				return -EINVAL;
 			}
-			append_preset_atom(preset, var, true);
+			err = append_preset_atom(preset, var, true);
+			if (err)
+				return err;
 			off += n;
 		}
 	}
···
 	void *tmp;
 	struct var_preset *cur;
 	char var[256], val[256];
-	int n, err, i;
+	int n, err;
 
 	tmp = realloc(*presets, (*cnt + 1) * sizeof(**presets));
 	if (!tmp)
···
 		return -EINVAL;
 	}
 	/* Remove trailing spaces from var, as scanf may add those */
-	for (i = strlen(var) - 1; i > 0; --i) {
-		if (!isspace(var[i]))
-			break;
-		var[i] = '\0';
-	}
+	rtrim(var);
 
 	err = parse_rvalue(val, &cur->value);
 	if (err)
···
 	if (err)
 		return err;
 	if (idx < 0 || idx >= barr->nelems) {
-		fprintf(stderr, "Array index %lld is out of bounds [0, %u]: %s\n",
+		fprintf(stderr, "Array index %lld is out of bounds [0, %u): %s\n",
 			idx, barr->nelems, array_name);
 		return -EINVAL;
 	}
···
 		}
 	}
 
-	return -EINVAL;
+	return -ESRCH;
 }
 
 static int adjust_var_secinfo(struct btf *btf, const struct btf_type *t,
···
 			break;
 		case FIELD_NAME:
 			err = adjust_var_secinfo_member(btf, base_type, 0, atom->name, sinfo);
+			if (err == -ESRCH)
+				fprintf(stderr, "Can't find '%s'\n", atom->name);
 			prev_name = atom->name;
 			break;
 		default: