xen-netback: make sure that hashes are not send to unaware frontends

In the case when a frontend only negotiates a single queue with xen-
netback it is possible for a skbuff with a s/w hash to result in a
hash extra_info segment being sent to the frontend even when no hash
algorithm has been configured. (The ndo_select_queue() entry point makes
sure the hash is not set if no algorithm is configured, but this entry
point is not called when there is only a single queue). This can result
in a frontend that is unable to handle extra_info segments being given
such a segment, causing it to crash.

This patch fixes the problem by clearing the hash in ndo_start_xmit()
instead, which is clearly guaranteed to be called irrespective of the
number of queues.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>