selftests/bpf: correctly move 'log' upon successful match

Suppose log="foo bar buz" and msg->substr="bar".
In such case current match processing logic would update 'log' as
follows: log += strlen(msg->substr); -> log += 3 -> log=" bar".
However, the intent behind the 'log' update is to make it point after
the successful match, e.g. to make log=" buz" in the example above.

Fixes: 4ef5d6af ("selftests/bpf: no need to track next_match_pos in struct test_loader")
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240820102357.3372779-3-eddyz87@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/test_loader.c

@@ -522,7 +522,7 @@ static void validate_msgs(char *log_buf, struct expected_msgs *msgs,
 		if (msg->substr) {
 			match = strstr(log, msg->substr);
 			if (match)
-				log += strlen(msg->substr);
+				log = match + strlen(msg->substr);
 		} else {
 			err = regexec(&msg->regex, log, 1, reg_match, 0);
 			if (err == 0) {