ipv6: Fix handling of LLA with VRF and sockets bound to VRF

[ Upstream commit c2027d1e ]

A recent commit allows sockets bound to a VRF to receive ipv6 link local
packets. However, it only works for UDP and worse TCP connection attempts
to the LLA with the only listener bound to the VRF just hang where as
before the client gets a reset and connection refused. Fix by adjusting
ir_iif for LL addresses and packets received through a device enslaved
to a VRF.

Fixes: 6f12fa77

 ("vrf: mark skb for multicast or link-local as enslaved to VRF")
Reported-by: Donald Sharp <sharpd@cumulusnetworks.com>
Cc: Mike Manning <mmanning@vyatta.att-mail.com>
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>