udp: fix SO_BINDTODEVICE

udp: fix SO_BINDTODEVICE

Damir reported a breakage of SO_BINDTODEVICE for UDP sockets.
In absence of VRF devices, after commit 13e05d8b1a31 ("net:
ipv4: add second dif to udp socket lookups") the dif mismatch
isn't fatal anymore for UDP socket lookup with non null
sk_bound_dev_if, breaking SO_BINDTODEVICE semantics.

This changeset addresses the issue making the dif match mandatory
again in the above scenario.

Reported-by: Damir Mansurov <dnman@oktetlabs.ru>
Fixes: 13e05d8b1a31 ("net: ipv4: add second dif to udp socket lookups")
Fixes: 3a032f2b5488 ("net: ipv6: add second dif to udp socket lookups")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>