net: dsa: fix panic on shutdown if multi-chip tree failed to probe

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

net: dsa: fix panic on shutdown if multi-chip tree failed to probe

DSA probing is atypical because a tree of devices must probe all at
once, so out of N switches which call dsa_tree_setup_routing_table()
during probe, for (N - 1) of them, "complete" will return false and they
will exit probing early. The Nth switch will set up the whole tree on
their behalf.

The implication is that for (N - 1) switches, the driver binds to the
device successfully, without doing anything. When the driver is bound,
the ->shutdown() method may run. But if the Nth switch has failed to
initialize the tree, there is nothing to do for the (N - 1) driver
instances, since the slave devices have not been created, etc. Moreover,
dsa_switch_shutdown() expects that the calling @ds has been in fact
initialized, so it jumps at dereferencing the various data structures,
which is incorrect.

Avoid the ensuing NULL pointer dereferences by simply checking whether
the Nth switch has previously set "ds->setup = true" for the switch
which is currently shutting down. The entire setup is serialized under
dsa2_mutex which we already hold.