ipv6: Check RTF_LOCAL on rt->rt6i_flags instead of rt->dst.flags

In my earlier commit:
653437d0 ("ipv6: Stop /128 route from disappearing after pmtu update"),
there was a horrible typo.  Instead of checking RTF_LOCAL on
rt->rt6i_flags, it was checked on rt->dst.flags.  This patch fixes
it.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Cc: Hajime Tazaki <tazaki@sfc.wide.ad.jp>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>