kern: fix auditing of ptrace(2) syscall requests

`error` here is the return value of syscall_thread_enter() rather than
the syscall itself, so the committed audit records do not reflect
reality.  This is less harmful than them recording an error when the
operation actually succeeded, but it could still possibly be used to
throw off IDS techniques with things like bsmtrace.

Approved by:	so
Security:	FreeBSD-SA-26:45.audit
Security:	CVE-2026-49426
Reviewed by:	des, kib, markj, csjp
Differential Revision:	https://reviews.freebsd.org/D57847

sys/kern/kern_sig.c

@@ -2766,7 +2766,7 @@ ptrace_syscallreq(struct thread *td, struct proc *p,
 	td->td_errno = nerror;
 
 	if (audited)
-		AUDIT_SYSCALL_EXIT(error, td);
+		AUDIT_SYSCALL_EXIT(tsr->ts_ret.sr_error, td);
 	if (!sy_thr_static)
 		syscall_thread_exit(td, se);
 }