virt_wifi: implement ndo_get_iflink

->ndo_get_iflink() is useful for finding lower interface.

Test commands:
    ip link add dummy0 type dummy
    ip link add vw1 link dummy0 type virt_wifi
    ip link show vw1

Before:
    9: vw1: <BROADCAST,MULTICAST> ...
After:
    9: vw1@dummy0: <BROADCAST,MULTICAST> ...

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Link: https://lore.kernel.org/r/20200305090636.28221-1-ap420073@gmail.com


Signed-off-by: Johannes Berg <johannes.berg@intel.com>