xen/netback: free already allocated memory on failure in xen_netbk_get_requests

[ Upstream commit 4cc7c1cb7b11b6f3515bd9075527576a1eecc4aa ]

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>