netdev-linux: Avoid RTM_GETQDISC bug workaround on new-enough kernels.

netdev-linux: Avoid RTM_GETQDISC bug workaround on new-enough kernels.

Otherwise we can't detect classless qdiscs.

This has no useful effect for the currently supported qdiscs, which all
have classes, but it makes it possible to add support for new classless
qdiscs.

This suddenly makes netdev-linux complain about qdiscs it doesn't know
about (e.g. pfifo_fast), which isn't too useful, so this commit also
demotes that INFO message to DBG level.

Reported-by: Jonathan Vestin <jonavest@kau.se>
Signed-off-by: Ben Pfaff <blp@nicira.com>