xen-netfront: use __pskb_pull_tail to ensure linear area is big enough on RX

I'm slightly concerned by the "only in exceptional circumstances"
comment on __pskb_pull_tail but the structure of an skb just created
by netfront shouldn't hit any of the especially slow cases.

This approach still does slightly more work than the old way, since if
we pull up the entire first frag we now have to shuffle everything
down where before we just received into the right place in the first
place.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: xen-devel@lists.xensource.com
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>