xen/netback: don't do grant copy across page boundary

Fix xenvif_get_requests() not to do grant copy operations across local
page boundaries. This requires to double the maximum number of copy
operations per queue, as each copy could now be split into 2.

Make sure that struct xenvif_tx_cb doesn't grow too large.

Cc: stable@vger.kernel.org
Fixes: ad7f402a

 ("xen/netback: Ensure protocol headers don't fall in the non-linear area")
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>