perf ftrace: Use atomic inc to update histogram in BPF

It should use an atomic instruction to update even if the histogram is
keyed by delta as it's also used for stats.

Cc: Gabriele Monaco <gmonaco@redhat.com>
Link: https://lore.kernel.org/r/20250227191223.1288473-3-namhyung@kernel.org


Signed-off-by: Namhyung Kim <namhyung@kernel.org>