net: ipv6: Dont add RT6_LOOKUP_F_IFACE flag if saddr set

741a11d9 ("net: ipv6: Add RT6_LOOKUP_F_IFACE flag if oif is set")
adds the RT6_LOOKUP_F_IFACE flag to make device index mismatch fatal if
oif is given. Hajime reported that this change breaks the Mobile IPv6
use case that wants to force the message through one interface yet use
the source address from another interface. Handle this case by only
adding the flag if oif is set and saddr is not set.

Fixes: 741a11d9 ("net: ipv6: Add RT6_LOOKUP_F_IFACE flag if oif is set")
Cc: Hajime Tazaki <thehajime@gmail.com>
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>