xen-netback: fix extra_info handling in xenvif_tx_err()

xen-netback: fix extra_info handling in xenvif_tx_err()

Patch 562abd39 "xen-netback: support multiple extra info fragments
passed from frontend" contained a mistake which can result in an in-
correct number of responses being generated when handling errors
encountered when processing packets containing extra info fragments.
This patch fixes the problem.

Signed-off-by: Paul Durrant <[email protected]>
Reported-by: Jan Beulich <[email protected]>
Cc: Wei Liu <[email protected]>
Acked-by: Wei Liu <[email protected]>
Signed-off-by: David S. Miller <[email protected]>