Revert "xen: netback: remove redundant xenvif_put"

Revert "xen: netback: remove redundant xenvif_put"

This reverts commit 13ace9398bbbab7b81cb213d0db8cc057b6bd883.

This change is incorrect, as per Jan Beulich:

====================
But this is wrong from all we can tell, we discussed this before
(Wei pointed to the discussion in an earlier reply). The core of
it is that the put here parallels the one in netbk_tx_err(), and
the one in xenvif_carrier_off() matches the get from
xenvif_connect() (which normally would be done on the path
coming through xenvif_disconnect()).
====================

And a previous discussion of this issue is at:

http://marc.info/?l=xen-devel&m=136084174026977&w=2

Signed-off-by: David S. Miller <davem@davemloft.net>