ipvlan: fix dev_id creation corner case.

In the last patch da36e13c ("ipvlan: improvise dev_id generation
logic in IPvlan") I missed some part of Dave's suggestion and because
of that the dev_id creation could fail in a corner case scenario. This
would happen when more or less 64k devices have been already created and
several have been deleted. If the devices that are still sticking around
are the last n bits from the bitmap. So in this scenario even if lower
bits are available, the dev_id search is so narrow that it always fails.

Fixes: da36e13c ("ipvlan: improvise dev_id generation logic in IPvlan")
CC: David Miller <davem@davemloft.org>
CC: Eric Dumazet <edumazet@google.com>
Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>