xen-netback: use default TX queue size for vifs

Do not set netback interfaces (vifs) default TX queue size to the ring size.
The TX queue size is not related to the ring size, and using the ring size (32)
as the queue size can lead to packet drops.  Note the TX side of the vif
interface in the netback domain is the one receiving packets to be injected
to the guest.

Do not explicitly set the TX queue length to any value when creating the
interface, and instead use the system default.  Note that the queue length can
also be adjusted at runtime.

Fixes: f942dc25 ('xen network backend driver')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Acked-by: Wei Liu <wei.liu@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>