netdev-linux: Fix self-deadlocks in traffic control code.

htb_parse_qdisc_details__(), which is called with the netdev mutex, called
netdev_get_mtu(), which tried to reacquire the mutex and thus deadlocked.
This commit fixes the problem and similar problems in
htb_parse_class_details__() and hfsc_parse_qdisc_details__().

Bug #19180.
Reported-by: Dhaval Badiani <dbadiani@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>