bpf: set 'changed' status if propagate_liveness() did any updates

Add an out parameter to `propagate_liveness()` to record whether any
new liveness bits were set during its execution.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250611200836.4135542-6-eddyz87@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>