ipv6: Refactor __ip6_route_redirect

Move the nexthop evaluation of a fib entry to a helper that can be
leveraged for each fib6_nh in a multipath nexthop object.

In the move, 'continue' statements means the helper returns false
(loop should continue) and 'break' means return true (found the entry
of interest).

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