virtio_net: Pass gfp flags when allocating rx buffers.

Currently, the refill path for RX buffers will always allocate the
buffers as GFP_ATOMIC, even if we are in process context.  This will
fail to apply memory pressure as the worker thread will not contribute
to the freeing of memory.

Fix this by changing add_recvbuf_small to use the gfp variant allocator,
__netdev_alloc_skb_ip_align().

Signed-off-by: Mike Waychison <mikew@google.com>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>