selftests/bpf: Fix timer_start_deadlock failure due to hrtimer change

selftests/bpf: Fix timer_start_deadlock failure due to hrtimer change

Since commit f2e388a019e4 ("hrtimer: Reduce trace noise in hrtimer_start()"),
hrtimer_cancel tracepoint is no longer called when a hrtimer is re-armed. So
instead of a hrtimer_cancel followed by hrtimer_start tracepoint events, there
is now only a since hrtimer_start tracepoint event with the new was_armed field
set to 1, to indicated that the hrtimer was previously armed.

Update timer_start_deadlock accordingly so it traces hrtimer_start tracepoint
instead, with was_armed used as guard.

Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Tested-by: Mykyta Yatsenko <yatsenko@meta.com>
Acked-by: Mykyta Yatsenko <yatsenko@meta.com>
Link: https://lore.kernel.org/r/20260415120329.129192-1-shung-hsi.yu@suse.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>