net: napi: Avoid gro timer misfiring at end of busypoll

net: napi: Avoid gro timer misfiring at end of busypoll

When in irq deferral mode (defer-hard-irqs > 0), a short enough
gro-flush timeout can trigger before NAPI_STATE_SCHED is cleared if the
last poll in busy_poll_stop() takes too long. This can have the effect
of leaving the queue stuck with interrupts disabled and no timer armed
which results in a tx timeout if there is no subsequent busypoll cycle.

To prevent this, defer the gro-flush timer arm after the last poll.

Fixes: 7fd3253a7de6 ("net: Introduce preferred busy-polling")
Co-developed-by: Martin Karsten <mkarsten@uwaterloo.ca>
Signed-off-by: Martin Karsten <mkarsten@uwaterloo.ca>
Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Joe Damato <joe@dama.to>
Link: https://patch.msgid.link/20260506090808.820559-2-dtatulea@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>