ipv6: Only call rt6_check_neigh for nexthop with gateway

Change rt6_check_neigh to take a fib6_nh instead of a fib entry.
Move the check on fib_flags and whether the nexthop has a gateway
up to the one caller.

Remove the inline from the definition as well. Not necessary.

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