net: dql: Optimize stall information population

When Dynamic Queue Limit (DQL) is set, it always populate stall
information through dql_queue_stall().  However, this information is
only necessary if a stall threshold is set, stored in struct
dql->stall_thrs.

dql_queue_stall() is cheap, but not free, since it does have memory
barriers and so forth.

Do not call dql_queue_stall() if there is no stall threshold set, and
save some CPU cycles.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://lore.kernel.org/r/20240411192241.2498631-4-leitao@debian.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>