selftests/rseq: Fix: Fail thread registration when CONFIG_RSEQ=n

When linking the selftests against a libc which does not handle rseq
registration (before 2.35), rseq thread registration silently succeed
even with CONFIG_RSEQ=n because it erroneously thinks that libc is
handling rseq registration.

This is caused by setting the rseq ownership flag only after the
rseq_available() check. It should rather be set before the
rseq_available() check.

Set the rseq_size to 0 (error value) immediately after the
rseq_available() check fails rather than in the thread registration
functions.

Signed-off-by: Mathieu Desnoyers <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

tools/testing/selftests/rseq/rseq.c

···
 		rseq_flags = *libc_rseq_flags_p;
 		return;
 	}
-	if (!rseq_available())
-		return;
 	rseq_ownership = 1;
+	if (!rseq_available()) {
+		rseq_size = 0;
+		return;
+	}
 	rseq_offset = (void *)&__rseq_abi - rseq_thread_pointer();
 	rseq_size = sizeof(struct rseq_abi);
 	rseq_flags = 0;