net/sched: netem: validate slot configuration

net/sched: netem: validate slot configuration

Reject slot configurations that have no defensible meaning:

  - negative min_delay or max_delay
  - min_delay greater than max_delay
  - negative dist_delay or dist_jitter
  - negative max_packets or max_bytes

Negative or out-of-order delays underflow in get_slot_next(),
producing garbage intervals. Negative limits trip the per-slot
accounting (packets_left/bytes_left <= 0) on the first packet of
every slot, defeating the rate-limiting half of the slot feature.

Note that dist_jitter has been silently coerced to its absolute
value by get_slot() since the feature was introduced; rejecting
negatives here converts that silent coercion into -EINVAL. The
abs() can be removed in a follow-up.

Fixes: 836af83b54e3 ("netem: support delivering packets in delayed time slots")
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260418032027.900913-5-stephen@networkplumber.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>