net: shaper: protect from late creation of hierarchy

In the Linux kernel, the following vulnerability has been resolved:

net: shaper: protect from late creation of hierarchy

We look up a netdev during prep of Netlink ops (pre- callbacks)

and take a ref to it. Then later in the body of the callback

we take its lock or RCU which are the actual protections.

The netdev may get unregistered in between the time we take

the ref and the time we lock it. We may allocate the hierarchy

after flush has already run, which would lead to a leak.

Take the instance lock in pre- already, this saves us from the race

and removes the need for dedicated lock/unlock callbacks completely.

After all, if there's any chance of write happening concurrently

with the flush - we're back to leaking the hierarchy.

We may take the lock for devices which don't support shapers but

we're only dealing with SET operations here, not taking the lock

would be optimizing for an error case.