net/ipv6: Defer initialization of dst to data path

Defer setting dst input, output and error until fib entry is copied.

The reject path from ip6_route_info_create is moved to a new function
ip6_rt_init_dst_reject with a helper doing the conversion from fib6_type
to dst error.

The remainder of the new ip6_rt_init_dst is an amalgamtion of dst code
from addrconf_dst_alloc and the non-reject path of ip6_route_info_create.
The dst output function is always ip6_output and the input function is
either ip6_input (local routes), ip6_mc_input (multicast routes) or
ip6_forward (anything else).

A couple of places using dst.error are updated to look at rt6i_flags.

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