selftests/bpf: Strengthen timer_start_deadlock test

Strengthen timer_start_deadlock test and check for recursion now

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260204055147.54960-5-alexei.starovoitov@gmail.com

Author: Alexei Starovoitov

tools/testing/selftests/bpf/progs/timer_start_deadlock.c

@@ -31,7 +31,6 @@ SEC("tp_btf/hrtimer_cancel")
 int BPF_PROG(tp_hrtimer_cancel, struct hrtimer *hrtimer)
 {
 	struct bpf_timer *timer;
-	static bool called = false;
 	int key = 0;
 
 	if (!in_timer_start)
@@ -42,13 +41,9 @@ int BPF_PROG(tp_hrtimer_cancel, struct hrtimer *hrtimer)
 
 	/*
 	 * Call bpf_timer_start() from the tracepoint within hrtimer logic
-	 * on the same timer to make sure it doesn't deadlock,
-	 * and do it once.
+	 * on the same timer to make sure it doesn't deadlock.
 	 */
-	if (!called) {
-		called = true;
-		bpf_timer_start(timer, 1000000000, 0);
-	}
+	bpf_timer_start(timer, 1000000000, 0);
 	return 0;
 }