gro: Flush GRO packets in napi_disable_pending path

When NAPI is disabled while we're in net_rx_action, we end up
calling __napi_complete without flushing GRO packets.  This is
a bug as it would cause the GRO packets to linger, of course it
also literally BUGs to catch error like this :)

This patch changes it to napi_complete, with the obligatory IRQ
reenabling.  This should be safe because we've only just disabled
IRQs and it does not materially affect the test conditions in
between.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>