eth: mpnic: add a NAPI depletion check

The Rx path can wedge if page pool allocations fail in
__mpnic_fill_bdq(). The problematic sequence is: page pool allocations
fail, bdq rings are empty, device receives packet from the network, but
drops it for lack of buffer space and does not raise interrupt,
mpnic_poll() does not run, so page pool allocations are not retried.

To break out of this cycle, we can add a depletion check to our service
task. The depletion check checks if posted buffers are under a low
watermark, and raises a completion queue interrupt, so that mpnic_poll
will run on the corresponding napi vector.

A note about the lock free reads: We don't bother synchronizing the ring
head and tail reads with mpnic_poll(), because a false positive signal
would simply raise a spurious interrupt scheduling mpnic_poll(), and a
false negative signal will result in a retry, where if mpnic_poll() has
truly not been running, should then report accurately whether or not
posted Rx buffers are low.

Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
Reviewed-by: Harshitha Ramamurthy <hramamurthy@google.com>
Link: https://patch.msgid.link/20261001-linux-mpnic-v1-4-6422ee74233a@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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