ipv6: Fix __ip6_route_redirect

In my last commit (a3c00e46: ipv6: Remove BACKTRACK macro), the changes in
__ip6_route_redirect is incorrect.  The following case is missed:
1. The for loop tries to find a valid gateway rt. If it fails to find
   one, rt will be NULL.
2. When rt is NULL, it is set to the ip6_null_entry.
3. The newly added 'else if', from a3c00e46, will stop the backtrack from
   happening.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>