net: sit: Update lookup to handle links set to L3 slave

Using SIT tunnels with VRFs works fine if the underlay device is in a
VRF and the link parameter is set to the VRF device. e.g.,

    ip tunnel add jtun mode sit remote <addr> local <addr> dev myvrf

Update the device check to allow the link to be the enslaved device as
well. e.g.,

    ip tunnel add jtun mode sit remote <addr> local <addr> dev eth4

where eth4 is enslaved to myvrf.

Reported-by: Jeff Barnhill <0xeffeff@gmail.com>
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>