ipv6: fix NULL dereference in udp6_ufo_fragment()

This patch fixes the issue caused by ef81bb40
which is a backport of upstream 87c48fa3. The
problem does not exist in upstream.

We do not check whether route is attached before trying to assign ip
identification through route dest which lead NULL pointer dereference. This
happens when host bridge transmit a packet from guest.

This patch changes ipv6_select_ident() to accept in6_addr as its paramter and
fix the issue by using the destination address in ipv6 header when no route is
attached.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>