perf lock contention: Check race in tstamp elem creation

When pelem is NULL, it'd create a new entry with zero data.  But it
might be preempted by IRQ/NMI just before calling bpf_map_update_elem()
then there's a chance to call it twice for the same pid.  So it'd be
better to use BPF_NOEXIST flag and check the return value to prevent
the race.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Hao Luo <haoluo@google.com>
Cc: Song Liu <song@kernel.org>
Cc: bpf@vger.kernel.org
Link: https://lore.kernel.org/r/20231020204741.1869520-2-namhyung@kernel.org