net/sched: netem: only reseed PRNG when seed is explicitly provided

net/sched: netem: only reseed PRNG when seed is explicitly provided

netem_change() unconditionally reseeds the PRNG on every tc change
command. If TCA_NETEM_PRNG_SEED is not specified, a new random seed
is generated, destroying reproducibility for users who set a
deterministic seed on a previous change.

Move the initial random seed generation to netem_init() and only
reseed in netem_change() when TCA_NETEM_PRNG_SEED is explicitly
provided by the user.

Fixes: 4072d97ddc44 ("netem: add prng attribute to netem_sched_data")
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260418032027.900913-4-stephen@networkplumber.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>