xen-netback: Adding debugfs "io_ring_qX" files

This patch adds debugfs capabilities to netback. There used to be a similar
patch floating around for classic kernel, but it used procfs. It is based on a
very similar blkback patch.
It creates xen-netback/[vifname]/io_ring_q[queueno] files, reading them output
various ring variables etc. Writing "kick" into it imitates an interrupt
happened, it can be useful to check whether the ring is just stalled due to a
missed interrupt.

Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: xen-devel@lists.xenproject.org
Signed-off-by: David S. Miller <davem@davemloft.net>