Keep track of which Qdisc is currently root.  We need to implement
TC_SETUP_ROOT_QDISC handling, and for completeness also clear the
root Qdisc pointer when it's freed.  TC_SETUP_ROOT_QDISC isn't always
sent when device is dismantled.

Remembering the root Qdisc will allow us to build the entire hierarchy
in following patches.
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: John Hurley <john.hurley@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>