net: busy-poll: use ktime_get_ns() instead of local_clock()

[ Upstream commit 0870b0d8 ]

Typically, busy-polling durations are below 100 usec.

When/if the busy-poller thread migrates to another cpu,
local_clock() can be off by +/-2msec or more for small
values of HZ, depending on the platform.

Use ktimer_get_ns() to ensure deterministic behavior,
which is the whole point of busy-polling.

Fixes: 06021292 ("net: add low latency socket poll")
Fixes: 9a3c71aa ("net: convert low latency sockets to sched_clock()")
Fixes: 37089834

 ("sched, net: Fixup busy_loop_us_clock()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Mina Almasry <almasrymina@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20240827114916.223377-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>