xen-netback: don't de-reference vif pointer after having called xenvif_put()

When putting vif-s on the rx notify list, calling xenvif_put() must be
deferred until after the removal from the list and the issuing of the
notification, as both operations dereference the pointer.

Changing this got me to notice that the "irq" variable was effectively
unused (and was of too narrow type anyway).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>