netfilter: xt_qtaguid: work around devices that reset their stats

Most net devs will not reset their stats when just going down/up,
unless a NETDEV_UNREGISTER was notified.
But some devs will not send out a NETDEV_UNREGISTER but still
reset their stats just before a NETDEV_UP.
Now we just track the dev stats during NETDEV_DOWN... just in case.
Then on NETDEV_UP we check the stats: if the device didn't do a
NETDEV_UNREGISTER and a prior NETDEV_DOWN captured stats, then we treat
it as an UNREGISTER and save the totals from the stashed values.

Added extra netdev event debugging.

Change-Id: Iec79e74bfd40269aa3e5892f161be71e09de6946
Signed-off-by: JP Abgrall <jpa@google.com>