hv_netvsc: fix race in napi poll when rescheduling

hv_netvsc: fix race in napi poll when rescheduling

[ Commit d64e38ae690e3337db0d38d9b149a193a1646c4b upstream. ]

There is a race between napi_reschedule and re-enabling interrupts
which could lead to missed host interrrupts.  This occurs when
interrupts are re-enabled (hv_end_read) and vmbus irq callback
(netvsc_channel_cb) has already scheduled NAPI.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>