skb_alloc produces linear packets (using kmalloc()).  That can fail,
so should we fall back to making paged skbs.

My original version of this patch always allocate paged skbs for big
packets.  But that made performance drop from 8.4 seconds to 8.8
seconds on 1G lguest->Host TCP xmit.  So now we only do that as a
fallback.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>
Acked-by: NMax Krasnyansky <maxk@qualcomm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>