nexthop: Add support for lwt encaps

Add support for NHA_ENCAP and NHA_ENCAP_TYPE. Leverages the existing code
for lwtunnel within fib_nh_common, so the only change needed is handling
the attributes in the nexthop code.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>