gve: Make gve_turnup work for nonempty queues

gVNIC has a requirement that all queues have to be quiesced before any
queue is operated on (created or destroyed). To enable the
implementation of future ndo hooks that work on a single queue, we need
to evolve gve_turnup to account for queues already having some
unprocessed descriptors in the ring.

Say rxq 4 is being stopped and started via the queue api. Due to gve's
requirement of quiescence, queues 0 through 3 are not processing their
rings while queue 4 is being toggled. Once they are made live, these
queues need to be poked to cause them to check their rings for
descriptors that were written during their brief period of quiescence.

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>