gro_cells: remove spinlock protecting receive queues

As David pointed out, spinlock are no longer needed
to protect the per cpu queues used in gro cells infrastructure.

Also use new napi_complete_done() API so that gro_flush_timeout
tweaks have an effect.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>