gve: Avoid rescheduling napi if on wrong cpu

In order to make possible the implementation of per-queue ndo hooks,
gve_turnup was changed in a previous patch to account for queues already
having some unprocessed descriptors: it does a one-off napi_schdule to
handle them. If conditions of consistent high traffic persist in the
immediate aftermath of this, the poll routine for a queue can be "stuck"
on the cpu on which the ndo hooks ran, instead of the cpu its irq has
affinity with.

This situation is exacerbated by the fact that the ndo hooks for all the
queues are invoked on the same cpu, potentially causing all the napi
poll routines to be residing on the same cpu.

A self correcting mechanism in the poll method itself solves this
problem.

Tested-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Praveen Kaligineedi <pkaligineedi@google.com>
Reviewed-by: Harshitha Ramamurthy <hramamurthy@google.com>
Signed-off-by: Shailend Chand <shailend@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>