net/netvsc: fix underflow when Rx external mbuf

net/netvsc: fix underflow when Rx external mbuf

When rte_pktmbuf_attach_extbuf() is used, the driver should not decrease
the reference count in its callback function hn_rx_buf_free_cb, because
the reference count is already decreased by rte_pktmbuf. Doing it twice
may result in underflow and driver may never send an ack packet over
vmbus to host.

Also declares rxbuf_outstanding as atomic, because this value is shared
among all receive queues.

Fixes: 4e9c73e96e83 ("net/netvsc: add Hyper-V network device")
Cc: stable@dpdk.org
Signed-off-by: Long Li <longli@microsoft.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>