vxlan: fix vxlan_find_sock() documentation for l3mdev

Since commit aab8cc36
("vxlan: add support for underlay in non-default VRF")

vxlan_find_sock() also checks if socket is assigned to the right
level 3 master device when lower device is not in the default VRF.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: David S. Miller <davem@davemloft.net>