xen-netback: update ubuf_info initialization to anonymous union

The xen driver initializes struct ubuf_info fields using designated
initializers. I recently moved these fields inside a nested anonymous
struct inside an anonymous union. I had missed this use case.

This breaks compilation of xen-netback with older compilers.
>From kbuild bot with gcc-4.4.7:

   drivers/net//xen-netback/interface.c: In function
   'xenvif_init_queue':
   >> drivers/net//xen-netback/interface.c:554: error: unknown field 'ctx' specified in initializer
   >> drivers/net//xen-netback/interface.c:554: warning: missing braces around initializer
      drivers/net//xen-netback/interface.c:554: warning: (near initialization for '(anonymous).<anonymous>')
   >> drivers/net//xen-netback/interface.c:554: warning: initialization makes integer from pointer without a cast
   >> drivers/net//xen-netback/interface.c:555: error: unknown field 'desc' specified in initializer

Add double braces around the designated initializers to match their
nested position in the struct. After this, compilation succeeds again.

Fixes: 4ab6c99d ("sock: MSG_ZEROCOPY notification coalescing")
Reported-by: kbuild bot <lpk@intel.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>