ipv6: addrconf_f6i_alloc - fix non-null pointer check to !IS_ERR()

Fixes a stupid bug I recently introduced...
ip6_route_info_create() returns an ERR_PTR(err) and not a NULL on error.

Fixes: d55a2e37 ("net-ipv6: fix excessive RTF_ADDRCONF flag on ::1/128 local route (and others)'")
Cc: David Ahern <dsahern@gmail.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>