net: lwtunnel: Handle lwtunnel_fill_encap failure

Handle failure in lwtunnel_fill_encap adding attributes to skb.

Fixes: 571e7226 ("ipv4: support for fib route lwtunnel encap attributes")
Fixes: 19e42e45 ("ipv6: support for fib route lwtunnel encap attributes")
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>