net: add napi_busy_loop_rcu()

This adds the napi_busy_loop_rcu() function. This function assumes that
the calling function is already holding the rcu read lock and
napi_busy_loop() does not need to take the rcu read lock. Add a
NAPI_F_NO_SCHED flag, which tells __napi_busy_loop() to abort if we
need to reschedule rather than drop the RCU read lock and reschedule.

Signed-off-by: Stefan Roesch <shr@devkernel.io>
Link: https://lore.kernel.org/r/20230608163839.2891748-3-shr@devkernel.io


Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>