net: Fix vti use case with oif in dst lookups for IPv6

It occurred to me yesterday that 741a11d9 ("net: ipv6: Add
RT6_LOOKUP_F_IFACE flag if oif is set") means that xfrm6_dst_lookup
needs the FLOWI_FLAG_SKIP_NH_OIF flag set. This latest commit causes
the oif to be considered in lookups which is known to break vti. This
explains why 58189ca7 did not the IPv6 change at the time it was
submitted.

Fixes: 42a7b32b

 ("xfrm: Add oif to dst lookups")
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>