virtio_net: fix race in RX VQ processing

virtio_net: fix race in RX VQ processing

Michael S. Tsirkin says:

====================
Jason Wang reported a race in RX VQ processing:
virtqueue_enable_cb is called outside napi lock,
violating virtio serialization rules.
The race has been there from day 1, but it got especially nasty in 3.0
when commit c414c2bb4dc853a25b28f76bfe7d8cd7482714ae
"virtio_ring: support event idx feature"
added more dependency on vq state.

Please review, and consider for 3.11 and stable.

Changes from v1:
	- Added Jason's Tested-by tag
	- minor coding style fix
====================

Signed-off-by: David S. Miller <davem@davemloft.net>